Hello,
Currently, the upgrade is not possible, it hasn't been implemented yet. However, if you order Premium Multi, we can offer to refund you for the Premium Single plan - as long as it hasn't been a few months 🙂
In any case, the subscription is not tied to any contract, it won't be automatically renewed at the end of the period, and no fees will be charged. So, if you don't extend it at the end of the subscription period, it will be canceled.
Best Regards
MonsterASP.NET team