>> Even if they started with an unsalted password system, users can be migrated to the newer more secure system on next login. In thinking about this, I wonder

>> Even if they started with an unsalted password system, users can be migrated to the newer more secure system on next login. In thinking about this, I wonder if in that scenario you'd even have to wait until next login. You could just use the weak hash as the input to your salted hash function and keep a flag of whether or not you need to 'pre-hash' the password before using your v2.0 salted hash. As users log in you could replace slowly replace the double hashed entries with single salted hash versions and flip the flag.