Changing a user's name is fairly complicated... as a dozen tables are all matched up by the username.
Manually changing one can make things goofy.
But there's an easy change username hack which worked well for me under 6.1.1
Download it
here.
Then you can totally change the user's name... and everything will update.
If you want to do it with queries, you can look at the scripts in that download for all the queries that are used to change a username under those older versions.