Oh cool! Sounds like you have something successful going on 
How are you currently upgrading members? Just manually in stripe or some other way?
Maybe until this feature is developed, could users cancel their current plan via Discourse and then re-subscribe using the new plan they want to upgrade to? I know that isn’t the smoothest process but seems like it would work? I think they should be pro-rated appropriately when cancelling their current plan. Having them do it via Discourse would remove/add them to the appropriate groups that match their subscription level.