This is important:
Note that the cron only updates users who have been active in the last 72 hours, so there may be a delay for some users despite manually running the cron.
There's a code edit to get around that. Would be good to make that code change now to get done what you need to, then revert it back to normal.
I don't know where the code edit is. Like all good things, it was posted by Jake so I will see what I can find.
EDIT: It's ok, I have worked it out myself.
Back up this file: library/XenForo/CronEntry/UserGroupPromotion.php
And replace it with the one attached.