For the sake of those who are trusting Olympic and going for new receivers, let me say I REALLY, REALLY hope and pray it'll work out OK for you.
But FWIW, I'm with Quarterbore on this one.
After 10 years in the NFA game I've seen BATF reverse and revise so many rulings, and almost all are at the expense of NFA owners. And many of them have turned once-legal transferable MGs into contraband or post-samples, often years after the owners bought them at then-current transferable prices.
A recent example is the roundup of parts kits that, after they were sold, BATF decided they had been improperly demilled and thus had never ceased to be MGs. AFAIK, no one has been reimbursed for one cent of the cost -- the innocent buyers are just out the money.
Going back a few years, BATF did the same thing for M14 receivers that had been demilled and then built up again as match M1As that sold for $1k-$2k. BATF sat by while dozens were heavily advertised in magazines and sold, then went on a roundup. Again, AFAIK no one ever got a cent back, from either BATF or the manufacturer.
In the late '80s BATF decided to "re-date" several types of LMGs and HMGs. In these cases, they decided -- years after they had been sold to civilians -- that the subject MGs were completed after the May 19, 1986 cutoff. In these cases, BATF took pity and let the owners keep them .... but they were reclassed as post-samples and could only transfer in the future as post-samples, at post-sample prices. Folks lost tens of thousands in that one.
Also -- in 1988, IIRC -- BATF ruled that folks who had purchased registered AK sears but had not yet installed them in receivers could no longer do so, because to install them required drilling semi receivers as hosts. And in that case, BATF simply revoked the registration of the sears. The owners had an expensive, useless piece of metal. Again, no compensation.
Given the value of registered transferable M16 receivers today, I believe that if BATF decided to confiscate the replacement receivers, the owners would have a cause of action against Olympic. So yes, you could sue them. Of course, Olympic would have a defense that they were acting in good faith and that they truly believed their actions were legal. It would be interesting to see how it panned out in court ..... but someone would be paying lawyers a ton of money to find out.
IMHO, I would keep the most beat-up lower rather than run that risk. YMMV.