[ezveedub]

You can have one registered lower and use different uppers on it. You basically register the original SBR specs on your form 1 and once approved, assemble it. If you get rid of the original barrel length upper for another SBR upper, then you're supposed to update the ATF with the permanent change. As far as using multiple uppers, yes you can use them on the single registered lower. Not sure if some states want to consider constructive intent, but swapping uppers hasn't been illegal AFAIK on registered lowers.

Now having a upper with barrel less than 16" and NO registered SBR lower that's assembled with a rifle stock, that would be seen as constructive intent.

 

[ezveedub]

Buy a lower that is registered as 'Multi' if you can - I would also go as far as get a forged and one that already had ambi controls while your at it

You don't have to buy a multi caliber lower. Once you do a Form 1, you are the manufacturer registering the caliber.

 

[ezveedub]

Just make sure that the Form 4473 for the AR15 lower is marked "handgun" then you can do anything with it that you want if you follow the law. But once a "rifle" lower, always a "rifle" lower as far as AR15 lowers are concerned. Even if the 4473 is marked "handgun", if you build the lower into a "rifle", the lower is always considered a "rifle" under the law. How would they know, they would not unless they get their hands on it if it is a rifle then again if it is a pistol. Not likely to happen, but it is the law and we are expected to follow the law no matter how stupid it is.

If it's a stripped lower, the 4473 is marked "other", unless your state has other rules. Also, a stripped lower can be built into an AR pistol, and then a rifle and back to a pistol. If it were built into a rifle first, then it would be a SBR if built with a barrel less than 16" or a pistol afterwards.

 

[ezveedub]

Here's some info regarding stripped lowers when building pistols and rifles. It has the ATF references.
http://jerkingthetrigger.com/2013/11/20/ar-15-pistols-clearing-up-some-misconceptions/