No but the ATF has gone on record saying you must be able to return it to its dimensions stated on the form 1 or 4.
I'm just pointing out that engraving the barrel is legal, just don't forget that your reciver is a NFA item because that is where the s/n is. That is the registered part.
Probably won't be a big deal to put a 26 inch barrel on it and make it title one configurationas long as you still have the engraved short barrel handy with your paper work. but don't loan or sell it like that because the serial numbered part is still controlled by NFA.
Do you see what I'm getting at?