I've tried searching TV Tropes, and the closest trope I could find is Wig, Dress, Accent. Is there a larger trope that It's A Wig! could be a sub-trope of - the moment played for dramatic reveal, comedy, or quiet sympathy, where what was previously assumed by characters and/or audience to be a head of awesome hair turns out to be a wig? (Wig, Dress, Accent isn't quite it, as that's a recognised and established three-part formula for a character disguising themselves.)
Hello, Unknown Troper. You'll need to get known to lend a hand here.