I believe you would be legal if you do not put the preban upper on the postban lower.
BUT
I don't want to be a test case. If you don't own at least one preban lower somewhere, and if the ATF got a wild hair up their butt, they could charge you with intent. Even if you beat it you would be broke.
OTOH, the whole thing is a joke. No one has ever been charged with this law. It is just in the bag of tricks the ATF can use against someone if they want to. Like if they raid the house of a guy who is dealing in firearms and find a few illegally modified rifles they can add that charge.
You do what you want. For myself, I am squeeky clean. I have both pre and post ban rifles and not even alone in the privacy of my garage do I ever put a preban upper on a postban lower. I think the law sucks but I do obey it.