First of all, thanks for adding to the thread there. I was trying to make a couple good branch-off points, and I'm really glad someone else has added their bit so quickly.
But keep in mind that there are two Sarahs running around: one used to be Karyn, and one is a pair of underwear. The only one that's transforming is the one that's underwear, so she wouldn't be growing a penis, she'd be turning into boxers or something similar. She'd be a boy when she wakes up in the morning, granted, and then she'd have a penis, but it wouldn't be happening quite yet.
Again, thanks for adding. I'd like to see if you have somewhere you're going with Aaron :)