I don't know if it have been relased before, but there's an Elf TF in a christmas Ben ten Episode... I'll try to get some screenshots or a video and I'll show you! ;-)
?DIT : Here's a video in wich you can see Gwen tfed into an elf... Sorry, but it doesn't show the process. Even if I'm not sure if there's anny, it is still interesting, I think! ;-)