A simple plugin to change registered usernames.
I've used this plugin on multiple occasions on numerous sites and have never had an issue. It's seems to work well. (Update: As of 7/2/2013, I tested this on a WP Multisite, on a Super Admin account, and after changing the username on this account, I could no longer access the Network Admin area. Luckily I had another Super Admin account, so I was able to change it back. I'll notify the developer and see if this can be fixed.