Follow the below steps:
1. Go to the Register/Login tab of the plugin (Existing site).
2. Click on the Remove License Key button (This will free your license from existing instance). – Refer image below
3. Now you can use your license on the new site.
If the problem persists, please contact us at info@xecurify.com or Click here to contact us for support. It would be helpful if you could share your use case with us.