so I only recently noticed this but why are "mężczyna" and "dziewczynka" stressed on the first syllable?
They should be stressed on the second-to-last syllable as most Polish words. The voice here is a computer voice, it's hard to expect perfection from it.
Well, "zmienić się" is reflexive, so when something changes (itself) into something else. If someone changes something, that's just "zmienić".
On a different note, this verb doesn't have anything to do with changing one's clothes, which is "przebierać się" (imperfective) and "przebrać się" (perfective).
Correction: if you say "zmienić ubrania" implicitly, that's okay. But if you don't use the word for clothes, you can just say "przebrać się".