Now that the ban is history, I'd like to replace my post-ban FST on my Bushmaster with a pre-ban; i.e. put one on with a bayonet lug.  This isn't a critical update or anything, but I'm curious as to if it can/how it would be done.  Examining the rifle, I can't see any obvious way to remove the front sight assembly - it seems to be pinned/riveted and they look pretty sturdy.  Is this an easily-removable component?  Should I even try?  This is my first AR in a long time - it took me years to recover from the pure hate I had for the piece-of-crap M-16A1 the Corps gave me in 1983.

What's a FST?  Do you mean FSB? unless you know what you're doing, it is highly recommended to have the removal and installation of FSB done professionally by a smith.  

you can pull the pins but the problem is those pins are not drilled in the same place on any tower so you can't just swap them out.  you will need a new barrel and tower drilled together.  $150.00.