I’m fairly sure that when a group is configured to automatically set a trust level, that trust level is only applied if it raises a user’s trust level. It is not applied if it will lower the user’s trust level.
If it is a small number of users the best approach would be to manually set the user’s trust level from their admin page. If it is a large number of users, you might want to make the change from the Rails console. You would need some way of identifying which users should have their trust level lowered though.