It's as simple as right clicking the object in AD and selecting Rename.
Note that their home folder will *not* change automatically if it has a username variable in it, but it will work just fine and you can manually change it if you care. Likewise, any locally stored workstation profiles will still have the old folder structure (but still work fine).
If you're using Office 365 with DirSync there's a powershell command you have to run against the Exchange server to forcibly change to the new address if you need to update their email as well.
All in all it's a pretty painless process.