FWIW, I am not as certain as many seem to be that it is a certainty that all states will require the "fix" whatever it might be, to be completed in order to renew registration. For the most part, emissions tests check sensors, and if they all check out "ready" then you pass-- which all of these cars will do in their present condition. It seems to me that any rule targeting "affected" TDI owners will require new legislation at the state level. As far as I can tell, this is even true for California, because I am not sure that CARB has authority to target specific vehicle owners, rather than automakers. Because they will have to change the emissions test rules for these specific cars.
Nor is it very obvious, as many seem to think, that the EPA could force the states to do this. Through what mechanism? The EPA has zero control over auto registration at the state level. It is possible, as some have noted, that the federal government might attempt to force the states to act, in the same way that it forced the adopting of the Age 21 drinking age, or 55mph speed limit, etc. But that would likely require an new Act of Congress to support the EPA position. Have you checked in on the Congress lately? Not likely to support the EPA on anything, ever.
And even if it were, the "withhold funding" mechanism to force states to do XYZ, which produced the Age 21 drinking age and 55mph speed limit, among other things, was ruled unconstitutional by the Supreme Court just a year or two ago (the Obamacare decision).
It is going to be a very long while for a fix to be designed, proposed, and approved. It will then be another very long while before there is any mechanism to compel adoption of the fix by vehicle owners. And then yet another very long while to get the fix, because of the queue that would then exist at VW dealers.
I think it likely that I will put another 60 or 70k miles on the car before any of this could become an issue that I must deal with.