I have read a lot about this problem and I haven’t found any solution.
Is there a way to import user across databases from OJS 2.3 to OJS 3.1?
Apparently the only possible approach would be set up a backup of the current OJS live database, connect this database backup with the new OJS 3.1 system and run a script that update the database. This will update the database making it suitable with the new OJS system and keeping the information.