switch from free plan to paid plan with unsubscribe from the free plan

Could you add the new feature that when a user, subscribed to a free plan, subsequently purchases a paid plan, the subscription to the free plan was automatically canceled?

That sounds quite a sensible idea.

This would be very useful to me as well.

Even better if supplemented by the expiry of all paid plans then auto subscribes to the free plan.

How should the system cancel a free subscription? I guess you just want the subscription for that free plans to be expired?

For me, "expiring" the existing free plan when a paid plan is subscribed to sounds exactly the correct way to go. 

you could make it work in a similar way to "Joomla Group Setting" that when you buy a paid subscription, you are automatically unsubscribed from the free plan indicated in the reference field. I have attached an example below

Supplemented by the inverse, subscribe to the selected free plan (but doesnt have to be free) when the plan expires.

Care must be taken though if you keep multiple subscription records, e.g. by year, to ensure that the subscribe only happens if there are no remaining active plans of that type.

I think this type of logic is missing for all unsubscribe events currently, e.g. joomla group plugin. I added my own handling via the PHP plugin. This worked but isn't ideal.

For the users who need this feature: Unsubscribe mean delete the subscription for the original or just set the subscription to be expired ?

I don't need this anymore as I have stopped using a free plan within MP.

However if I were to need it in the future then I would have said in both cases.

For me just set the Free subscription to be expired, so we can see this in membership history.