JBS has a good idea of how -they- think but is incorrect.
if you keep the parts so that the SBR can be returned to the registered condition, no change of registration is required.
That is the right answer. That is what people across America do with their SBR and SBS to play around with the length. You can change what ever parts you want as long as you have the originals "readily available" to change it back to the way it is on paper. If you want to change the stock, just keep the CTR in the back of your safe with the stock wrench nearby.