It's not a silly question. Unfortunately, it's a question without a perfectly clear answer.
The two sides of the argument:
1. ATF Ruling 2011-4 (also known as US vs Thompson) clearly says you can go back and forth from pistol to rifle, as long as it started as a pistol. Part of the explanation includes the description "weapon made from a rifle".
2. When you file a Form 1, you're making a new firearm...just (usually) adopting the markings of the old one. So, you're no longer starting with a pistol, you now have an SBR, which cannot be converted to a pistol due to the "weapon made from a rifle" issue.
I used to firmly be in Camp 1. However, after the explanation from some members of Camp 2, I tend to agree with them. It's enough of a grey area that I'll never do it, and I'll never recommend anyone do it. That being said, Camp 1 still has a
lot of members.