Unfortunately, the Discourse Subscriptions plugin doesn’t currently support directly migrating users from one plan to another without manual intervention or requiring the subscriber to re-register under the new plan. Automating such transitions would likely require custom development.
The good news here is they don’t have to re-subscribe or be put on another plan really since you have already charged them once. But ya, unfortunately manually canceling each user is the only way through the UI to do what you are after. Then you can just move each user to the appropriate group they should be in as well.