You should check out some fics by bellatemple. She has a number of fics that deal with some transformation. This is her list (http://bellatemple.livejournal.com/193140.html#cutid3) of SPN fics. You can take a look at the summaries to see the transformation fics.

Have fun! :)

Here are all my transformation fics, just check the headers to see if they're gen or wincest
http://www.livejournal.com/tools/memories.bml?user=catnthecradle&keyword=Transformation&filter=all

This is my story http://yiyamika.livejournal.com/#post-yiyamika-2094 the other chapters are posted under my name. I hope u like it.