First of all make a backup of your users table and run this query:
Code:
UPDATE xf_user SET user_group_id = x WHERE message_count = 500;
Replace x with the group id that you want to move the users with 500 posts and xf_ with whatever prefix you are using for your db tables.
P.s. You might want to run the Rebuild User Caches as well after the query. btw.