[zappsan]

Execute the following query:

UPDATE vb_post SET userid=X WHERE userid=Y;

This will change all posts made by the user with the ID Y to be made by the user X.

UPDATE vb_post SET userid=X WHERE postid=Y

This will only set the post with the ID Y to be a post from the user with the ID X

Be sure to change vb_post, so it's using the prefix you use.
Also be sure to make a backup, before running a query.
(even though it should work since I tried it out on my testboard before)