Howto delete old plan

I have imported new CBSubs plans for all users on a website. They still have active old plan, but it is unpublished as it is not to be used anymore. How do I delete that plan? Right now it says it cannot be deleted because there are still users subscribed to it. The subscriptions runs out 31/12 - will it then be possible to delete the plan ?

You can't delete it if there are subscription rows for that plan otherwise all those rows can no longer reference it and would break upgradeability checks. Simply leave it unpublished.