You could change the primary user group with this query:
Code:
UPDATE user SET usergroupid = B WHERE usergroupid = A
(you need to add your table prefix to 'user' if you have one).
But changing the group might result in a change in user title or rank (which I think you can do by running "update user titles and ranks" in the "update counters" section of the Maintenance menu).
Another option might be to add a promotion from group A to group B and set the conditions so everyone qualifies (such as strategy "Posts" and 0 Posts), then run the "User Promotions" scheduled task (run it manually by pressing "Run Now" - if you wait for it to run itself it will only promote users who have been active recently).