They can only make these requirements at best via the commerce clause.
Thus, anything not in interstate commerce - like a gun you already own, cannot be required to be registered for you to continue to possess it within your state of residence.
The reason it doesn't violate ex post facto is that it does not make it a crime for you to have behaved differently the day the law goes into effect.
Say for example, it will be a crime to possess a semi-automatic rifle without a special tax stamp starting July 4th 2013.
Now, if you possess such a gun now, of course, with no tax stamp, then that is presently legal. You cannot be prosecuted for any period BEFORE July 4th 2013 that you possessed such a gun without paying the tax. You CAN be prosecuted for possessing such a gun without paying the tax on or after July 4th 2013.
Your actions prior to the law taking affect are and will forever remain - legal. You cannot be prosecuted for them. But perhaps CONTINUING to engage in them, might mean you run afoul of the NEW law.
But, as I said, since there is no interstate commerce, and no other provision of the Constitution being implemented here, then her bill can't do anything about guns already owned.