What parts of a pre ban "high capacity" magazine can be swopped out with new production parts? I'm guessing the follower/floor plate; but what about the spring and body?
At some point, if you replace enough parts, you have a new old pre-ban magazine. (A happy result, eh?)
Unless you have some secret, cheap, source, finding pre-ban standard capacity mag bodies may be almost as expensive as buying a whole pre-ban mag from some place like www.glocktalk.com or www.gunbroker.com
Every post ban bushmaster mag I've ever seen has been dated on the body. Best bet is to stick to C-Products mag bodies as replacements. No C-Products bodies are dated...
Under the old Federal ban, it was the governments problem to prove that the mag was illegal. Who knows what would happen if this ever became a problem in MA, but as far as I know, no one has ever been prosecuted/indicted, let alone convicted of an assault weapon ban violation in MA.