The day guys have female parts, and girls have male parts is the
day that guys will be known as girls, and girls will be known as
guys, and everything will remain normal.
So if that is the case, there really can't be a scenario whereby
guys have female parts, and girls have male parts, because we can
essentially say that it has already happened. Perhaps you were born
a guy with female parts, which leads to everyone thinking you are a
female, and so for the past few years you existed, you are a
female.
Bleh, I have no idea what I'm saying. 
