If one were to submit and get approval for a form 1 stating 300 blackout caliber with an 8" barrel, then later acquire a 11.5" 5.56 barrel, then forget they filed as a 300 blackout and sell that upper.......what steps if any would need to be taken?
Hypothetically of course
Doesn't matter, once you have stamp in hand you can change length and caliber
You can, but if it's a permanent change, ie selling the upper that was associated with the original F1, you are supposed to inform the atf. I did this with an F1 suppressor that ended up being considerable shorter than when I filed the F1. They sent me back a thank you letter a few months later indicating that the registry had been updated. Do you need to do it, realistically? Probably not, but if you want to follow the letter of the law; then yes.
I like these "Hypotheticals". Why would you even bother? Just do it right. I waited until mine was approved for a 6 inch .50 beo. Then I wrote them a letter informing them I wanted to modify my sbr and requested 35-40 calibers and barrel lengths be added. Kept the original envelope with all the dates on it and put it back away along with my paperwork. It's easy to get approval. Everyone should get all ar15 sbrs approved for all calibers. Preemptive for any potential law changes.