As far as where Bigmiker21 got his information .... well, it was probably from ATF itself.
Here's the deal:
–– There is
no law against the owner of a registered SBR changing the barrel length once the Form 1 has been approved, even if that means he cannot return the registered item to the spex listed on the Form 1.
––ATF is required, by Congress and by federal law, to maintain the NFA Registry in as accurate a form as possible. That's why they "request" that you notify them in writing whenever you change calibers/dimensions/etc. They
must do so, because they are required to do so by federal law.
So can you be charged with a crime if you fail to notify them (or fail to maintain the parts necessary to return it to the configuration described on the Form 1, and listed in the NFA Registry)? No.
Would you potentially piss off the same jack-booted thugs who like to shoot pet dogs and pregnant women if you fail to comply with their requests? Ummm .....
If you want to sell the upper that meets the Form 1 spex and get another, just spend the ten minutes it takes to drop them a note with the new dimensions.