[James Birkett]

I would suggest a php script to inject the mysql names and overwrite the current usernames.. (or an sql query?). That means each username has to correspond to each current username however and it could be difficult. I can't see it being possible via the admin CP or anything else that is built-in, except sql database edits. Maybe you should back up your database, open it in a suitable program, sort it in the same order as is sorted in your excel spreadsheet (i.e. by e-mail, as your usernames are the e-mails right?), overwrite each one then re-inject the database.
I know that might be hard to understand - see if you understand it