Log in to your WP Tasty account.
Select "View Your Purchase History."
Click "View Licenses" for the purchase that contains the plugin.
Click "Manage Sites."
Click "Deactivate Site" on the old URL that will no longer use the plugin, and enter the new URL in the form. Click "Add Site" to save the information.
Once this is complete, you may continue to see the license prompt on your site for a while until the transient clears. You can force the transient to clear, however, by removing and re-adding your license.