Mello-Roos taxes are set at the time of the first sale and won’t change until they expire (i.e., if the assessed tax at 450K was an extra 2250/yr, even if someone later buys it at 250K, it’s still going to be 2250/yr).

As I understand it though, the base RE tax (1.1%ish) is reassessed at the sale price when the property changes hands. There might be an exception for REO properties, but I hadn’t heard of it before now.