Hi there, Richard! Thank for getting in touch. π
Good question. If the demo subdomain is still live, you can remove the license key from it by heading to Events > Settings > Licenses from the WordPress admin. Copy and delete it there (save your settings), then head over to the new site and add it there instead. Once the license key is entered in the new site, it should let you know it’s been verified (screenshot) and you can save your settings.
Once your license key is registered and saved in WordPress on the new site, the new domain will also display in your account here on this site (Account Central > License Keys).
Does that make sense? Please let me know. π
Cheers!
Geoff