Would it still be the same ar15 in the eyes of the ATF or would/could that be an element of constructing a new firearm?

Not interested in doing it. But say another ban similar to the 94' AWB happened with the exact same language, could guys that wanted to build a different gun from scratch and still legally retain all of the evil features cut the FCG and serial number portion out of a pre-ban ar15 lower receiver and weld it to a completely new designed gun lower and have it still considered the same ar15 per the ATF?

I trip over lower receivers when I open up the safe, I need some what if scenarios as to what I can do with them in the future

The established case law has been that if you change it to some different "category" of firearm, or substantially change the method of operation, or incorporate too much of a different firearm to the receiver, a new firearm has been created.

Whether you require a license to do so is dependent on your state laws and whether you are doing so for personal use or to offer in the channels of interstate commerce.

Whether such an action is prohibited without being a current SOT depends on how the BATFE ends up classifying the resulting newly-created firearm.

The guidelines are super shifty - nothing's set in stone.

No....

IF the 2nd Amendment was being followed you could do pretty much anything you wanted.

Realistically, the .gov will do whatever they want and at least one of the other branches of .gov will go along with it.

Don't ask don't tell. Worked real well between 94-04