I am only able to log in as local administrator (it won't let me log on as any other user since the DC is unavailable). This is a Windows 2003 R2 server. The "Change" button is greyed out in Control Panel --> System --> Computer Name tab, so I can't click it. The domain controller and the entire domain has been decommissioned and this one machine got overlooked.
The other answers I've seen on this site assume that the Change button is available.