I have an SBS question. Once a Form 1 has been processed and the tax stamp has been received and firearm cut to short length, what rules if any govern what can be done with it. Specifically I am interested in  cutting down a Stoeger Condor o/u 20 ga. I note that Buds sells a combo model that has a set of 20 and 12 ga barrels. I'm thinking I could cut down the 20 ga barrels and keep the 12 gauge unmodified. Cut the stock down to a pistol grip and buy an extra standard length stock to bolt on when I want to shoot the 12 ga. I understand that the ATF ASKS that you notify them if you make changes after you have an approved stamp, but I am uncertain if there is anything that prevents what I am suggesting. Thanks for any informed responses.

I'm not aware of anything to prevent what you're describing. It actually sounds like a great idea! 

ATF ASKS that you notify them if you make changes after you have an approved stamp

Only on items that have been permantly modified.  

Your situation is similar to the Lego set that is a AR.  Once a AR lower is SBR'ed, you can put whatever upper, in any caliber, your want to.

For your O/U shotgun, you could shorten any new barrel you buy for it, and run it with that SBS set up.

Think of it this way - a SBS or SBR configuation allows you to have either a shorty or full size barrel on it, with any caliber, or barrel lenght you choose.