I don't think that can be updated with a query alone. You'd need a script.
As the users may have various values like this for the U_Groups field:
Josh - -3-2-1-
JoeBloe -3-5-
Pete -3-5-8-
John -3-8-1
So while you can
UPDATE w3t_Users
SET U_Group = '-3-'
WHERE U_Username = 'Pete'
That will change -3-5-8- to -3- and blow out any other groups they were in.
You'd need a script to read the value of -3-5-8- and remove the -5 from whatever group they already were a part of.
Now if you know what groups that user should belong to - and it's only a couple of users you have to adjust, then the query above should help you. Just keep in mind you're updating the group field all together, not adding to or removing from it.
