Changing the version should not affect the license, unless you do an Uninstall first.
When the About box opens it tests to see if a valid license exists. If you click the 'Load License File' button it attempts to load a file, but that doesn't re-check the license. It is necessary to exit from the About box, then re-open it so the license check can be made. If it still says demo it means the license file was not valid.
The license is stored in the NfsMill.ini file which is in your application data folder. That is C:\Documents and Settings\{username}\Wizards\Mill. That file must be there, and not be set to read only. The installer tests to see if it exists, and does not replace it when you install.
Did you uninstall first?
Did you make some hardware change that might invalidate your license?