If you are using the iOS app, you can have any combination of subscriptions. 

Free, Pro Monthly, Pro Annual, and Domain+ subscriptions can be applied to any of your sites on the Universe iOS app. Each of your sites is able to have a different subscription designation.

To upgrade a free site, navigate to the Upgrade tab in your dashboard while viewing that site in your Sites List and select the subscription that fits your needs.

🌟On the Web App (Beta), you can currently only have one Domain+ subscription per account. We’re working on making it possible for you to upgrade existing free sites to Domain+!

🌟If you subscribe to additional annual subscriptions, you will be subject to Apple’s prorated billing structure. Read more about that here.

🌟To see a list of all of your current subscriptions, you can view them with Apple here.