Do you need a gun smith to pop out two pins and remove an upper from an AR15?
Absolutely not. It can be done in like 10 seconds.
It can be replaced in 10 seconds as well by a well trained monkey.
Yet that process (or procedure) is considered 'manufacturing' by the NFA. It however only applies if you are adding a shorter upper than 16".
Whether or not you purchased that upper and lower is irrelevant. You are the one converting it to fall under the NFA.
If you are the one that did the modification then you do a Form 1.