ATF's position is that you're Form 1 is an application to build a particular SBR. As such, it has an explicit caliber, barrel length, etc. on the moment when you build it and that is the information they want on the form.
As for having multiple uppers, the key is to have a legal way to assemble every lower you have in your possession. If the only legal way you have to assemble a spare lower is as an unregistered SBR, using one of your spare uppers, then you could be found guilty of possession of an unregistered SBR.
As long as you've got a 16"+ upper for every non-registered lower, you're golden.
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