I think the way I'd approach this is:

import all users into FusionAuth

At cutover time:

look at local database to see which password hashes had changed pull the user data from FusionAuth for each of these users delete the user re-import the user with the new password hash and the FusionAuth data, maintaining the same userId (if you provide the UUID, we'll use that)

I get that is an additional complexity, but hopefully that helps.