Thanks for writing in!
Of course, you can use the license to another site. Simply reassign the license code to your new domain and you are all set to validate the theme installed in the new domain. To validate X properly, please do the following:
1] Please log in to your APEX account and assign the license (https://theme.co/apex/licenses). You need to assign the license to your site using this URL: https://www.new-url.com/. You have to make sure that URL you just assign is the same as the one you have in your settings, Settings > General > Site Address
2] Go back to X > Validation, and insert the license code.
And please review the changes that we made on how you are going to validate your X theme because we made a big change in the process. Please check it out here: https://theme.co/apex/forum/t/setup-product-validation/55
Hope this helps.