Doing something progressive is probably the right way to go. I'm not sure how large a drop in property values merely announcing the policy would have. I'm also not sure how long a time window would be necessary. I think if the change is sufficiently gradual it might be possible to do so without compensation for either mortgage owners or property owners. Of course for that to be true it has to still make sense for current mortgage owners to pay their mortgages. I think that suggests a window size of larger than 25 years, but I'm not sure how much larger than that it would need to be.