On all the shows I've seen on the Discovery channel, if the person goes all the way with the surgery, they legally change status to whatever gender they have transitioned to. So let's say "Ken" transitions to "Barbie". His driver's license will now read "Female". If "Ken" had previously been a member of XYZ fraternity in college, I would think he would forfeit his membership. Otherwise, wouldn't it put XYZ fraternity in danger of legally not being a single-sex organization anymore?
Oh the things you'll respond to at 3:36 am after taking your teenage brother-in-law to the Harry Potter premiere!