I just made a request of a username change, but will that only be displaying, or when I logg inn too, and if I want to search for stuff related to me?

Example: *old username*:fav is still valid, or can I only go by *new username*:fav ?

I think they fixed it so all your information is also tied to the new username.
I used to have search comments related to all of my old usernames individually, and now I don't. Not sure if the same thing happened to favorites but I'd imagine it might not be a problem.

My old username returns no results for anything, so it looks like you'll need to use the new username when searching.

The only thing your old username will still work for is searching for yourself in comments, since comments are not updated with your old name. Well, comments you create will show as being created by your new username. But if someone types "Hey hatsjer" in a comment and you change your username to "johnny12", it's not going to update the comment to say "Hey johnny12" so you'll still have to search for "hatsjer" to find older comments directed at you.