I added a promotion - Greater than 2 posts promoted from default registered group to a custom group. I ran the task as well, but no members groups changed to. They all remained just registered.
Is group promotions not retroactive?
Not sure if it's retroactive, but you can also perform a query to achieve this.
What's a query and where to perform it:
http://community.mybb.com/thread-44380.html
Your query:
UPDATE `mybb_users` SET `usergroup` = 'x' WHERE `postnum` > '2'
Change x to the new group id.
I ended up manually doing it through ACP 'Views'... The promotion system now seems to working fine with new users reaching the quota. Thanks