Extra options in subscription plan

All plans were already set to "exclusive".

The prorate was turned off on all plans (it never gave an issue with renewing memberships before so it's always been turned off). I turned it on and tried again (in the new plan, I added a duration for one month for "free" just for testing). But the result is still the same. It should add a month to the expiration date of the already active plan, but instead it starts counting the month from now.

Prorating won't add time from 1 plan to another. Adding time only applies to renewals. When going from 1 plan to another prorating simply reduces the price of the plan you're upgrading into based off the remaining value of the plan you're upgrading from. There is no way to add the duration from Plan A to Plan B when upgrading to Plan B.