According to NATE IIRC if a copycat is not possessed before 10-1, then it's illegal to possess it after 10-1. I think that's what he postulated. Point being the lower isn't a copycat before 10-1. I believe putting a 7.5 inch upper on the lower after 10-1 would be prohibited.
IANAL.
What if it was built as an M4 replica before then?