I would de-register a purchase code from a test website, and use the code in new website. Unfortunately if I use the code, in new web site, the code it tells me that it is still registered.
I remember a website of envato, where put the purchase code and the code was deregistered … unfortunately I do not remember the site.
Envato does not have any system for managing purchase code registration. This is instead handled by authors with their own individual systems. You’ll need to reach out to the author of the theme for instructions.