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.

If you don't mind self-recs, my J2 BB this year is genderswap Leaving Footprints behind (http://cillab42.livejournal.com/45262.html)

Sam/Dean femslash: she's a real time girl (http://lazy-daze.livejournal.com/652168.html)(in my imaginary world) covers most of your points (less plot-y)

There's also Flood of Water (http://sevenfists.livejournal.com/83527.html), where Sam goes down on Girl!Dean,

And Rivkat's interesting double gender swap, Hating the Weather, (http://rivkat.livejournal.com/283633.html) in which Dee was always a girl and gets turned into a guy. If it's about the gender changing and not about the female body, for you.

I've written girl!Dean (http://tebtosca.livejournal.com/64396.html), and there's cunnilingus involved