Actually, the Custom Field Modified Subscription Duration can only be used for onetime payment subscription plan. It could not be used for recurring subscription
For recurring subscriptions, you will need to create different subscription plan, each plan with specific/fixed subscription lengths
I’d like to suggest adding a note beside the modify subscription duration field explaining that it supports one-time payment plans only and maybe adding a link to relevant documentation for further explanation.
Since the field allowed the edit and correctly extended the subscription dates inside membership pro it appeared to work for recurring plans too. The limitation only became apparent when I checked the billing interval in PayPal. A visible warning would help other site owners catch this before creating subscriptions with mismatched billing terms.
