Antecedents of 'game-changer'
For many years people have discussed the effects on various sports and pastimes of introducing innovations in rules, equipment, personnel, or tactics in terms of their potential to "change the game." For instance, "Concerning Rules and Regulations," in The Yale Literary Magazine (January 1905) offers this commentary on football:
Among the body of undergraduates there is a certain amount of discontent with the game [of football] as it is played. But this discontent arises almost entirely, as has been said before, from the too-indiscriminate attacks—for many of the articles which have appeared, whatever was the intention with which they were written, are neither more nor less than attacks on football. Much of the remainder of the discontent arises from the fact that not every one would be satisfied with perfection, and perfection is not easy of attainment. However, there undoubtedly is a residuum of honest belief that football can be changed for the better. Of course it can. It isn't this belief that this leader disagrees with, but the manner in which people go about the attempt to change the game, and the desire to make the change to sweeping. Football is as popular as any sport—to repeat again—professional, amateur or strictly collegiate. This fact does not seem to show any widespread discontent with the game among the individuals of either public or any pressing general demand for radical change. Any radical change is fully as likely, under these conditions, to destroy the popularity of the game as to increase it.
And "Effect of Inventions upon Sport and the Trade," in Hardware (December 25, 1906) contains this discussion in connection with golf:
While contemplating the efforts of our modern inventors to place on the market an article that contains so-called improvements over the old styles, the thought occurs that possibly those interested in the manufacture of these goods do not realize what changes they are bringing about in the sport or pastime in which they are used. In fact, writers on certain branches of sport are protesting that their games are already in danger of being spoiled even now by the introduction of utensils that have a too revolutionary effect. Take golf, for example, which was made the subject of an article by an ingenious writer in a periodical publication. The contention is that the constant improvement in the construction of golf balls and clubs is rapidly changing the game from a pleasant contest of skill to mere cross country pedestrianism. Whether or not the game of golf is being "spoiled," it certainly is being noticeably changed.
It stands to reason that as discussions of "changing the game" continued over the decades, someone would resort to "game changer" sooner or later.
Early occurrences of the term 'game-changer' used in a literal sense
The William Safire column mentioned in Josh61's question is from the New York Times of September 12, 2008. The paragraphs relevant to the question of when the term was first used literally and figuratively are as follows:
In early 2003, White House officials began telling journalists "nuclear weapons are a game-changer" and to transform Iraq would be "a geopolitical game-changer." By June of the next year, President Bush made it official, with definition attached: "A free Iraq in the heart of the Middle East is going to be a game-changer, an agent of change."
When did the game begin? I first tracked it to a logical source in sports. The Washington Post had a 1982 baseball reference: "Singleton hit his game-changer ... fair by eight yards" and a decade later described football's Desmond Howard as "a game-breaker and a game-changer." But when I set that etymological Inspector Javert—Ben Zimmer of www.visualthesaurus.com— on the trail, he noted the adoption by business motivators of the sports metaphor, including a prescient 1995 reference in The Wall Street Journal to the Internet as "a real game-changer." Casting a wider net, he came up with an origin beyond sport, in playing cards: The Atlanta Constitution’s "Bridge Forum" of June 29, 1930, frowned on attempts to improve the game of bridge: "Seldom are the game-changers idle."
I can't find a copy of the 1930 Atlanta Constitution article online, but it appears to be using game-changers in the literal sense of "ones who are changing the game."
Safire cites a 1982 Washington Post story in which Ken Singleton of the Baltimore Orioles had a "game-changer"—that is, a hit that effectively changed the particular game in the Orioles' favor. Here "game" refers to a specific contest—a particular baseball game, rather than the game of baseball itself. The description Safire cites from a decade later of football player Desmond Howard as "a game-breaker and a game-changer" refers to Howard's ability to break open a close game with kickoff return, a punt return, or a long reception. Thus "game-breaker" here refers to a particular player, not to a particular play.
Even more suggestive is this item from Inside Sports, volume 3 (1981), which also appears in Thomas Boswell, Why Time Begins on Opening Day (1984) [combined snippets]:
Find the 6-2, 200-pound super jock, put him at third and you've got the game-changer. The in-their-prime prototypes are [Mike] Schmidt (6-2, 203), [George] Brett (6-0, 200) and [Buddy] Bell (6- 2, 185)—all drip greatness and charisma. They occupy the idealized place in the game that was reserved for fleet power-hitting centerfielders 25 years ago when debate raged over the merits of [Mickey] Mantle, [Willie] Mays, and [Duke] Snider.
The usage here is similar to the Desmond Howard usage: the focus is on a player who is capable of altering the outcome of individual games. But there is also a sense here that the player is altering the broader game itself. Just as a certain type of tight end altered the way U.S. football was played in the late 1970s (which the article earlier alludes to), the emergence of a type of agile, power-hitting third-baseman is altering the way successful baseball teams are being put together and managed. So here we have an instance where game-changer refers in part to changing the sport itself and not just a particular contest.
I note, however, that the first two editions of Paul Dickson, The Baseball Dictionary (1989/1999) do not to have an entry for "game-changer," strongly suggesting that the word/phrase was not, as of 1999, an established, baseball-specific term of art.
'Game-changers' and 'game-changing'
We've seen that a game-changer is a game-changing event, player, or (in a figurative sense) idea. From this, it follows that instances of the term game-changing might be important precursors of the noun game-changer. But Google Books search results turn up few matches for game-changing as an adjective prior to the late 1990s. The exception appears in Charles Savage, "Comments" (a response to Stanley Krippner, "Consciousness-Expansion and the Extensional World"), in Et Cetera (December 1965) [combined snippets]:
The difficulties of all these models is that they defeat the purpose of the psychedelic experience, which is to liberate one from the old models. Thus the semantic model rescues us from one procrustean bed and forces us into another. But this is inevitable since one needs models with which to communicate. And Krippner's model seems as good as any that has been constructed. As a corollary explanation Krippner adduces the game model of behavior: "The psychedelic substance enabled these men [prisoners] to break through word-games which had to a large extent been responsible for their downfall." I would question that this explanation is either accurate or sufficient. Using this very game model I would predict a high rate of recidivism, given sufficiently long follows-ups. Though the prisoner may be momentarily freed from the game of cops-and-robbers, the other members of his team of robbers and the opposing team of cops will continue to play the game, society will assist at the metagame, and the prisoner will be again enthralled by the cops-and-robbers game.
I would submit that a much better illustration of the game changing potential of the psychedelic experience is the Harvard controversy itself which Krippner does not mention. The psychedelic revolution nearly succeeded in changing the game structure at Harvard. Its failure seems to me a classic instance of (1) a failure to follow out the logical implications of one's own model, and (2) of confusing one's model with the reality which it purports to explain. They confused the game model of behavior with behavior itself. They fell into the very trap against which Krippner warns—taking "concepts for data ... words for actual things." They became prisoners of their own model.
The context here is evidently the highly theoretical world of "the game model of behavior." (The "Harvard controversy" mentioned in the second paragraph, by the way, refers to experiments conducted in 1960 and 1961 investigating the effects of psilocybin and other hallucinogens on Harvard undergraduates.)
In addition, one early instance of game-changing used as a noun appears, in Esquire magazine, volume 82 (1974), reproduced in George Leonard, The Ultimate Athlete [combined snippets]:
The duel finished, Brand began a short talk on the theory of New Games. Still breathing hard, he explained the rules of game-changing to about a hundred of us gathered around him on the gym mat.
"You can't change the game by winning it, losing it, refereeing it, or spectating it. You can change the game by leaving it. Then you can start a new game. If it has its own strength and appeal, it may survive. Most likely it won't. In either case, you will have learned something about the process of game-changing and the particular limitations imposed on us by certain games."
Although the idea of "game-changing" here is unusually theoretical, it clearly accommodates the idea of a game-changer as a person responsible for leaving a game (in the sense of a sport or pastime) and starting a new one.
First definite use of 'game changer' in a figurative sense
As Mari-Lou A points out in her answer, the earliest Google Books match for game changer used in a purely figurative way is from Catherine Hayden, The Handbook of Strategic Expertise: Over 450 Key Concepts and Techniques Defined, Illustrated, and Evaluated for the Strategist (1986) [combined snippets]:
Game Changer: an industry event that changes the competitive rules of the game. GAME CHANGERS are usually EVOLUTIONARY PROCESSES that change the COMPETITIVE FORCES in an industry and its INTRA-INDUSTRY STRUCTURE or indications that the competitive forces could be changed by an innovative competitor.
The notion of "changing the game," in the sense of altering the fundamental way a sport or pastime is played, has been discussed in published texts going back more than a hundred years. The adjective "game-changing" appears at least as far back as 1965, in a highly analytical context involving the game model of behavior; and game-changer used in a literal sense may go back as far as 1930 (in the context of the card game bridge, according to research noted by William Safire) and certainly at least as far back as 1981 (in the context of a discussion of revolutionary changes in the use of certain positions on the field in baseball and U.S. football).
The first confirmed figurative use of game changer in Google Books search results involves business jargon (reported in 1986), where it refers to an innovation that alters the competitive landscape in a particular industry or market.