I was very irritated to see that upgrading to 3.0 resulted in posts being divided into the old and new users.
My unifying user was subscribed again to all forums. Once this happened, I moved the posts from the old user to the new. After that, a ctrl-shift-r to properly reload, and all posts belong to the main user.
Here is the SQL command.
UPDATE `Posts` SET `UserId`='ID_YOU_WANT' WHERE `UserId`='OLD_ID'This doesn't appear to have any negative consequences on attachments and the few other things I checked.
After backing up and confirming this, I deleted the old users.
I hope that helps others a bit.