Form 1 SBR questions

So I am considering doing a form 1 on one of my lowers, it is currently marked as multi caliber, my understanding is the it has to be marked with a specific caliber such as 5.56, would I be able to shoot a 300 BLK upper on it after it was approved?

I guess the biggest question really is, can I swap uppers of different length and caliber on the lower after it is SBR'd?

I know I can't take it out of state without notifying the ATF

TIA

Changing Your SBR Upper with a Different Caliber Than the Registered Lower - National Gun Trusts

https://www.nationalguntrusts.com/bl...CTYuoAL8qlLO93

FPNI

thanks for the link, now I know what is involved.