Quoted:
your gun is an SBR and you can change the configuration at will. you don't even have to notify them legally.
As tony_k has noted in the past, the ATF has a legal responsibility to keep the NFA database as accurate as possible. If you
permanently change your barrel length by selling off or otherwise disposing of the original barrel, it's a good idea to
jhelp the people who issue our tax stamps by sending them a letter.
You can have umpty hundred different uppers with different barrel lengths, but if you dispose of the original barrel (or dispose of all barrels that length) it's a good idea to let ATF know-by letter, as tony noted.
And no, you don't have to wait for a reply (which could take a very long time). Just let 'em know. The definition of an SBR is a rifle with a barrel "less than 16" in length," so any short barrel is fine. But your "default" configuration is supposed to be accurate, even if they don't twist your arm to make you let them know about it.