My understanding: when you are in the draft stage, you can select any release (start/end), plan the work within the capacity planner, then publish it. Are you saying that you did the "planning" (chose portfolio items/group allocations) and then you are trying to switch between releases? Because if so, then at that stage you are the one setting which release you are planning for before you publish the plan. For more than one release, you'd create a new draft/publish a new plan.