I bet there will be an executive order to remove ex post facto once they put a limit on it or outlaw it so we can be taken away for being the horrible tobacco users we are. HAHA!
The SCOTUS has already ruled that ex post facto does not apply to taxes. That is, if they wanted to, the Feds could institute a retroactive tax on all unconsumed tobacco. I cannot imagine how they would execute this on your home stash, however they could certainly hit retailers on unsold stock.