I also think it's unlikely, but California did at one point revoke a tax exemption and applied that decision retroactively, expecting interest on unpaid taxes that the law at the time said you didn't owe. As I recall, this wasn't considered ex-post-facto simply because being fined for under paying taxes isn't the same thing as convicting someone as a crime, so the rule of law doesn't have to work the same way. You can take any constitutional protection and someone will come up with an argument in their head for why it doesn't apply to things they don't like.