[kh99]

You can change the usegroup using a query, like:

UPDATE user SET usergroupid=X WHERE fieldY='Japan'

where of course X is the id of the new group for Japan users, and Y is the id of your custom profile field. But I think if you do it that way, it's possible that not everything gets updated as it should when a usergroup is changed. What you might do is make a "temporary Japanese" usergroup, use the above query to move all "Japan" users to the temporary group, then use "Move/Prune Users" to move then from the temporary group to the Japan group.