[SOLVED] Lower plans available to upgrade

HI,
i have a subscription system with 4 plans.
The lower one is a free lifetime plan and while the others are to scale: the next includes the previous one (1.Spirito Libero, 2. Weekender 3. Cicloviaggiatore 4. Esploratore).
In the settings I entered that the upgrade is only possible if the plan is more expensive but then in my account (with the highest plan) I see that it is also possible to upgrade to the free plan and to one of the other two (not the third one) - see attached picture.
 
Unfortunately, one of my subscribers also upgraded from a higher plan to a lower one by mistake... how can I fix this?
thanks

Add upgrade conditions to your plans that prevent upgrading if they're already actively subscribed to a more expensive plan.

The upgrade based off price takes into account discounts as it's purely a price comparison check it can sometimes be unreliable depending on your configuration. The conditions however will allow you to strictly control the upgrade flow. So for example edit your free plan and add conditions that prevent upgrade if actively subscribed to one of the paid plans.

The below for example should work.

First static condition: This condition must be NOT be met to allow upgrades:
Any of the following plans subscriptions: SELECT_CHEAPER_PLANS_HERE
Must be in state: Active

You may also need to select in the state parameter "Upgraded to other" as well.

Thanks Krileon,
it solved the problem!