If a Windows Embedded Standard 2009 OS is built with a trial license key, it will expire after some amount of time (either 30 or 120 days, I'm not sure which). It's very easy to give the system a valid permanent license key before the trial expires, but once the license key expires the system won't boot at all (even after adjusting the BIOS system clock to a time before the license expired and booting without an internet connection).
However, it's still possible to access the file system by booting from a secondary media, e.g. a bootable USB stick. Is there a way to manually provide a valid license key after the original trial license has expired, or does the entire image need to be rebuilt?
EDIT: Note that according to a post on social.msdn, this is not possible and the image must indeed be rebuilt. No evidence for why the change cannot be made retroactively is given, however.