Easy to replace post ban front sight housing when/if the AWB dies?  Also, KKF's extended A2 FS for a BM M4 clone would allow a bayo lug to be functional on said rifle?

TIA

It's not really overly difficult, but I would strongly recommend having a gunsmith do it for you (KKF's a good choice) just to make sure the front sight is properly aligned.  I've done several myself and those bloody pins are a pain in the ass to get in!

If the AWB isn't replaced ), I'd probably suggest for you to just send your upper to KKF and have Kurt install a new front sight post, thread the barrel and install the FS all at once.  Be ready for a wait, though, I'm sure he's going to be busier than ever if everything goes well on the AWB issue.