Ok.  Can the fixed position buttstocks be 'un-fixed' back to a collapsible stock on a during the ban Bushmaster M4 without buying any special parts?  Is it legal to do this?

Personally, the collapsible stock is the only AWB part I would care to have on an former post-ban rifle.  The bayonet and flash hidder I can do without.  Thanks.

Short answer: No.  

Long answer: You need to unpin the stock, drill holes for the different positions, get the parts that actually let the lever work (right now it's just a hex nut) AND even with all that done, the receiver extention has about 2 inches of extra metal that would have to be cut off.  With all that, the Bushmaster telestyled stock uses a rifle buffer and spring so you wouldn't be able to collapse the stock fully, because the newly sized receiver extention would still be too long.