All from the UBB control panel.
The first item to do is add a member group called members.
Then set permissions the same as users.
Then in registration settings change the default group from users to members.
Then all new registrations will be in the members group.
Then the other suggestions in this topic will move existing people to users and change any reference from users to members.
Or use the database tools to change all users to members. All the usual disclaimers apply; make sure you have a good backup that is restoreable, take your forum offline so no changes can occur while you're manipulating the db, etc., etc.
Run this sql query to find out what the group id is for members:
select * from (your board's prefix)_GROUPS
Once you have the ID for Users (4) and Members (?) run this update query:
update (your prefix(_USER_GROUPS set GROUP_ID=='?" where GROUP_ID='4'
That will change all the current Users to Members but will not affect Administrators or Moderators.