Hey! I have a self rec for 'turned omega'... It's really short; but hopefully it'll meet your criteria?
Here's the link in case you're interested-

http://sanshal.livejournal.com/5613.html

*hug*

For genderswitch, here are two slow transformation Dean stories (including sort of dark Sam) off the kink meme:
Improving Nature (http://spnkink-meme.livejournal.com/33949.html?thread=8075933#t8075933) - Sam is obsessed with the idea of making Dean a girl
Girly Girl (http://spnkink-meme.livejournal.com/4278.html?thread=2228918#t2228918) - Sam gets Dean on steroids so he becomes very feminine

Also, I haven't read this one, but I think it fills your first request:
Big Brother Omega (http://spnkink-meme.livejournal.com/68645.html?thread=22322725&#t22322725) - Omegas aren't born that way. They're made. Every child born to an Alpha/Omega parents is born Alpha. Daily exposure to the hormones present in Alpha semen turns an Alpha into an Omega.

Try this one:

Alpha Falling (http://sylsdarkplace.livejournal.com/33389.html) by sylsdarkplace: This is an Alpha/Beta/Omega AU in which Alphas Jared Padalecki and Jensen Ackles have fought for control of the construction industry in Dallas with dirty deals and underhanded actions throughout their twenties. Theirs is a world where aggression and dominance are valued above all else, and in fear of losing his position as head of his company, one of them will bring down his foe with one ruthless act that tears both their lives apart and forever binds them together.

It's pretty dark and I would recommend to read the warnings carefully. Not an easy read for me but so worth it! The way gender roles are explored is really brilliant.

Just thought of another one:

Hunter's Lullaby (http://etoile-etiolee.livejournal.com/6358.html) by etoile_etiolee: An encounter with a witch leaves Dean in a disturbing predicament. The relationship he shares with his brother will be put to test, and everything he thought he could control in his life will be changed forever.

This isn't A/B/O but mpreg. After an encounter with a witch Dean is pregnant. The magic slowly changes his body to accommodate the child. And well, you can imagine Dean's reaction... ;)

That´s great, I have searched for it!

Shifting Equilibrium (http://spnkink-meme.livejournal.com/68407.html?thread=22496567&%20#t22496567)
Jared's turned into an omega werewolf by alpha Jensen. Dark!

The Adventures of Dean and His incredible Shrinking Penis (http://bitter-crimson.livejournal.com/551333.html) by bitter_crimson (slow genderswap)
Summary: "So," he says to Bobby, pants around his ankles, "what do you think?"

Bobby, with a remarkably straight face, studies the display for a few minutes, then grunts and takes a swig from his flask. "Well," he says, "it's not Koro, that's for sure, although it seems to be something similar. Why didn't you mention that your testicles were shrinking, too? Unless that's," he coughs, "normal."
Comment: Not very long, but good and kind of fun :)

Getting Soft (http://hoodie-time.livejournal.com/426817.html) by nwhepcat(slow genderswap)
Summary: Dean should have known better than to ask for the happy ending. Set in the first winter after Sam's departure for Stanford.
Comment: An amazing fic! It also has a sequel Learning Curves (http://hoodie-time.livejournal.com/493875.html), this is one of those verses where I'm constantly hoping for the author to write more :)

An Unexpected Turning (http://vivhasarifle.livejournal.com/?skip=10&tag=verse%3A%20unexpectedturning) by vivhasarifle
Summary: Dean is a member of a pack of lynx and much to his surprise he discovers he is a carrier. Fearing the reactions of his pack, he runs, and encounters an interesting jaguar with piercing blue eyes.
Comment: Dean isn't born as an alpha, but he doesn't expect to turn into an omega either. Really good :) BTW: This is a mpreg-fic, so if you don't like that, don't read it.

Love In the Time of Change (http://yourgracewasted.livejournal.com/7840.html) by yourgracewasted
Comment: By mating with the alpha angel Castiel, human Dean turns into an angelic omega. One of my favorites, that I re-read all the time, but this is also mpreg :)

The Other Side of Time (http://bluetoastfics.livejournal.com/25373.html) by darth_firefly
Summary: Dean's first case after Sam's departure for Stanford would also be his last. Running into a Fae catcher, Dean's taken out of the human realm into the next. One of the members of Oberon's court, a fey named Orrick takes Dean into his home - and sets to making the young man his consort. However, first the powerful lord intends to cure Dean of the sorrows of his former life that have been growing like poison in his body since he was four.
Comment: This might not be your thing since it basically deals with Dean turning into a fae, but that change also entails some significant changes to his genitalia. This is also mpreg, btw.


Edited 2013-05-15 08:23 pm (UTC)