In general, if you 'Bubba' enough you MIGHT be de-C/R ing the weapon and POSSIBLY rendering it illegal to exist in this country if imported as a C/R. The situation arrises mostly with some imports ONLY allowed in owing to the 'original condition' of them as C/R military arms that would otherwise be prohibited (under 1989 reg) AW's. The example best known are the Yugo SKS's with bayos, night sights, grenade launchers; if you start popping those things off, go further, and put the deal into a plastic nifty modern stock---you're out of line at SOME POINT. You really have to know case-by-case what you are doing and how the weapon acquired the C/R status. I would follow my gut and not get too 'cute' about what I was doing or try to work the corners out of some regulatory 'angle' by finding a path. Is anyone going to 'care' is what we all want to know many times. I am NOT going to offer any examples of 'cutesy' C/R behaviors I've heard of as they are NOT good ideas. 'Nuff said, if something 'feels' like it might be over the line it very well might be to someone.
But then also be cautioned that in some places C/R status meshes with gears of State/Local laws. You have to keep your own council at days end; we ALL do.
Best advice; leave things mostly as is UNLESS by 'Bubba'ing you are trying to substantially re-create or repair them to original condition or that the mod can be justified for safety (ie. known parts issues that cause a modification to design but have been well estabvlished for a particular weapon. Again with the Yugo SKS example, the ATF allows for a spring return to be added for safety to the firing pin although the original desgn doesn't include this)..
iF YOU ARE really worried just call the BATFE and ask them. Will you 'get in trouble' is a question I really wouldn't guess at in fairness about much of ANYTHING involving C/R and MANY issues involving the finer points of firearm laws and regs in general.