Welcome aboard Robert.
Basically you're title/question is also the answer "Once a machinegun, always a MG"
No dealer samples are transferable, with the exception that when an
SOT goes inactive, he/she may if they desire, keep any left-over
tranferable NFA firearms and Pre-May '86 dealer samples however, if
the SOT does decide to keep a Pre-May '86 dealer sample, it can NEVER
be transfered to ANYONE but another SOT, LEO agencies, etc.,
Post guns are not transferable to non-SOT's in any form, period.
Exchanging or removing NFA parts out of an already classifed NFA
firearm and replacing the parts with all semi parts, does not change
it's classification in any way.
Exception: NFA "Registered Bolts" may be used in non-NFA firearms, as in these cases the bolt itself is registered as the NFA weapon, therefore, the bolt may be used in any firearm that will accept it.
Once an MG, SBR, etc., ALWAYS an MG or SBR. In fact, once an SOT, ALWAYS an SOT.
To others out there that may be a bit more caught up lately than I am, please feel free to interject.
Hope this helps with your questions, feel free to ask anytime, it's always better to ask and be safe, than to assume and be sorry. NFA isn't something to be taking chances with, and although to my knowledge the above is accurate info, it is still technically an opinion and not to be construed as positive fact. For positive fact and legal proof it is always best to contact NFA's technology branch and get your answers in writing from them if you do need legal info on something you may be considering.
Did I also mention that NFA shooting is a real BLAST! It's loads of fun for