Are you allowed to use different upper receivers on your SBR, as long as they're the same caliber?

You can use different uppers. It doesn't matter what caliber the upper is. The caliber, barrel length, and overall length needs to match the paperwork when the SBR is assembled the first time.

Yes and who would check anyway?  Nobody ever checks.

You can put any upper you want on your registered SBR lower.  I have a 9mm upper with a 10" barrel, a 5.56mm upper with a 14.5" barrel (it's an M4 upper with KAC rail and so on), and a 300 Blackout upper with an 8" barrel.  They all run fine - and legally - on that same lower.

The data on your Form 1 is the "original configuration" of the SBR.  With an AR, that's just your starting point.

Boom. Feel the same way too