Every subscription is based on a certain domain and can't be changed for a new domain. This is because historical data and progress are no longer valid for the new domain, as the new website will have different backlinks, different social KPIs and different keyword rankings. This is not our policy, it's Google and every search engine's! As they crawl every website, they will count your website with a new domain as a new page. What you need to do is to cancel the old subscription and create a new one with the new domain.
Don't worry about what you have already paid, it won't mean any extra cost for you. We'll do it internally. And if you are concerned that your progress is going to be lost, it won't be like that. Say you have optimized a page for a given keyword, well if this remains the same our crawler won't open again that certain task.