A friend of mine has a theory that changing the emphasis from one syllable of a word to another never really affects the "core" pronunciation. So for instance, consider the word umbrella.
The emphasis usually falls on the second syllable: um · BREL · la. But if you decided to emphasize the first syllable instead: UM · brel · la, that doesn't really change what those syllables sound like; it just gives greater force to one of them. The first syllable is still a short-U sound in either case; it doesn't suddenly become a long-U sound because of an emphasis change.
Is my friend's theory correct? Or are there any counterexamples to this, where changing the emphasis actually does alter the fundamental sound of a syllable? (e.g. a legitimate emphasis change -- not a mispronunciation -- that changes a long-A sound to a short-A sound, for instance)