There is still a chance and a passive compatibility tolerance...
By default login/password_downward_compatibility = 1. So you have both hashes. You can migrate the users via BAPI_USER_CREATE1 (BCODE and PASSCODE are sent, but without guarantee that they will work) and set the hashcodes in the target and then set the compatibility to 2 or lower for a while in the new target system.
The old BCODE hashes are certainly synchronizable if the old BCODE still means anything to the user. You can also plan migrations...
Better choice than above is to migrate the users and give them a service to reset their own passwords or something for the admin to do it for them and send the pwd to the smtp address ( which you will have migrated as well). It is good to have a fail-over plan anyway even if you do not often have migrations.
Cheers,
Julius