Thanks. Should I completely drop both databases and replace with the back ups or import the additional records?
No, thats different for both databases.
1) ISPConfig database: drop it and replace it with the db from the backup.
2) mysql.mysql Database: export only all ispcsrv* users from all tables in the mysql database and import them in the database on the new server.
Please make backups of both databases on the new server before you start, so that you can restore them in case that something goes wrong