At about three o'clock, the professor transitioned from the topic of communication, to the topic of political correctness.
or
At about three o'clock, the professor transitioned from the topic of communication to the topic of political correctness.
At about three o'clock, the professor transitioned from the topic of communication, to the topic of political correctness.
or
At about three o'clock, the professor transitioned from the topic of communication to the topic of political correctness.
The comma is superfluous in my opinion because it breaks up the flow of the sentence more than necessary. In this case, the "to" portion still forms part of the main clause and is a vital part of the sentence.
See this rule from Purdue University's rules of comma usage:
- Do not use commas to set off essential elements of the sentence, such as clauses beginning with that (relative clauses). That clauses after nouns are always essential. That clauses following a verb expressing mental action are always essential.