NMN does its magic mostly or entirely by delivering NR to cells. The science is clear that NMN cannot directly enter most (or perhaps any) cells, so the NMN must first be degraded to NR or NAM outside the cells before it can enter the cells and be built back up into NAD.

Happily for NMN users, there are circulating enzymes that do that. But you don't have to depend on those enzymes, because you can deliver the NR directly. Not only is that more efficient, it is often less expensive, and I have seen no science indicating that it is less effective.

If the FDA enforces its ban on the sale of NMN in the US, then NMN users will have to switch. But the fact that the switch leaves them no worse off because NR is available, means that the harm from the ban falls mainly on NMN suppliers, not consumers, which is one reason of several reasons why the FDA is likely to enforce the ban.