IMHO there is not much "grandfathering" going on if existing owners of newly NFA'd items have to undergo the background check, fingerprinting, positive ID, and LE certification rigmarole.
I expect that numerous coastal and certain Midwest urban areas (the usual suspects, you know where they are...) will quickly pass restrictive state or local laws disallowing possession, which will give large numbers of current owners legal standing to argue that the law is effectively an unfairly punitive uncompensated taking, thereby potentially throwing the whole thing into a legal never-never land.
As I've posted in previous threads, I'm more concerned that it will evolve into a sort of "NFA Lite" that largely leaves existing owners alone but enacts punitive taxes on the sale of the items.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak