In the customer portal, the customer should be able to view all the available plan and then switch from the existing plan to other plan.
Say there is Silver, Gold Platinum plans and a customer is a subscriber of Gold plan, then on the Change plan page, show all the available plan with the customers subscribed plan i.e. Gold plan selected. The customer selects the other plan and clicks submits, on confirmation switch the customer to other plan calling the Upgrade/Downgrade API in the Subscription system.