The bayonet lug must be ground off. The spot can be touched up with TouchUp Blue and will not look bad.
A comp or brake, NOT a FLASH hider needs to be added and permanently silver soldered, welded, pinned, etc. Any device you put on the muzzle may not have a diameter of 22 mm, in which case it is considered a grenade launcher.
The Cav Comp from Cavalry Arms is a good one. Works well keeping the muzzle jump down, does not seem any noisier than the original flash, does not look much different from the A2 hider. It screws on, and then you drill through the hole in the bottom about 1/16", and drive in a blind pin. Then you weld over the pin. This can be ground smooth and fixed with touchup blue, too, for a neat job. I like this as not much heat gets to the barrel itself. Just a quick dab of the rod and the hole is welded over. This is one of the least damaging, and objectionable comps around.
These two changes, to remove the flash hider and make the threads inaccessable, and to remove the bayo lug, will make this upper legal on any lower.