I had a molly I thought was a male once, but then it turned out female due to an injury. I've also had a female get an injury there before that made it look like a boy. lol Sometimes it can take a while for that part to go back to normal after an injury. My fish are also easy-going, but sometimes the girls do NOT want it, and will get into it with a male while the male is sniffing around her back end, resulting in a rather damaging bite. I assume, if it WAS an injury, it's the same for mollies and platies, they are told apart the same way.
Other than that, I don't know. I've never heard of a molly or platy going from male to female, but when they're young they can go from female to male. But that's below the five month range or so. I haven't heard of it going the other way around. lol