Some recs:

http://causeways.livejournal.com/85586.html : Sam is willing to do whatever it takes to get Dean out of the deal. [20k]

http://archiveofourown.org/works/178612 : When Dean disappears on a hunt, the last thing Sam expects is the help of another psychic in trying to find him. The search doesn't go well, and when Dean walks back into his life, it's with news that tears the world out from under them both. [51k]

http://annella.livejournal.com/375598.html : Summary: Dean turns into a girl. Him and Sam have teh secks.

http://archiveofourown.org/works/4720 : A story in 21 parts, featuring an Ancient Greek curse, an unexpected metamorphosis, adventures in pool sharking, numerous shots of tequila, a nun outfit, zombies, angels, demons, kidnappings, startling discoveries about old acquaintances, massage, a game of strip poker, girl-on-girl action, girl-on-boy action, and boy-on-boy action. [57k]

Take a look here (https://pinboard.in/u:spnkink_meme/t:kink:genderbending/) and here (http://sd-recs.livejournal.com/1235.html), too.