This search has led me to realise that I am missing a story.

It is in the "Sam rescues Dean from Hell" trope with a heck of a twist. Sam alters Dean's body into a duplicate of his, thinking by doing so he can change Dean's soul JUST enough that it's not recogniseable. It works and they live on -- but Dean notices his preferences are changing as well. And at the end of the story, Bobby mistakes Dean for Sam and a panicked Dean looks in the mirror and realises that he can't see himself in his own eyes anymore, all he can see is Sam.

I really would like to find this and share it with the original searcher. Thanks for any help!