Gun Control & RKBA

The government made a law banning X unless X was registered by a date. If X was not registered on that date, it is now illegal.

I think there's a legal argument that could be made, but it's not ex post facto. It's more like a government operating a system that would be legal if it was run competently but not if it was run incompetently, either deliberately or accidentally. For example, it might be legal under current Constitutional law to require that in order to obtain an abortion, a woman would need to obtain an ultrasound at a government-operated clinic. If all the clinics were functional, that might pass Constitutional muster. If, on the other hand, the Texas government had this law and then all of the government clinics "had" to be shut down for months/years for repairs, that (hopefully) would not be Constitutional.