You can effectively rename a user account but as this is messing wit the system tables I would suggest dropping and adding the user back. But for those of you who really want to know how here it is.
First off I tested this and it works fine renaming a user.
However this is directly modifying the system tables and setting the configuration to do this.
I make no guarantees that you could not possible cause yourself an issue.
And if you do make these kinds of changes they are yours to deal with if failure occurrs.
Note: I did find out that you can do this while the user is logged in. They just will have to use the new login name after they logout and come back. Also you will need to be a member of the server admin role to run this.
Ex. usp_RenameLogin 'Test', 'TestLogin'
Like the one posted before, the use of the SP is like this: proc_alerts 1,18. This will show the first 18 alerts on the system. OR proc_alerts 20,35 will show from 20 to 35. This lets you manage the list without letting go out of the screen.