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.
no subject
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.