Randy is correct that you can wait until a MIRE event. There are also "trip wires" like knowledge of a map change. Like Randy said, if you don't have a Life of Loan service, you may not know that a map change effected a property you have as collateral. I've seen many examiners that expect you to know. If you do have knowledge of a map change, it's difficult to argue "I didn't know". It really depends on how much knowledge you have of the map changes, your borrowers and their collateral.
I don't believe you need to do a massive scrub. IOW, you don't need to create knowledge. However, if you know a property is now in a SFHA that wasn't before the map change, I don't believe you can stick your head in the sand and ignore it waiting for a MIRE event.