If I upgrade to a new plan why does my old plan still exist?

We want it so our members can only be subscribed to a single plan at a time - how do we set this? We have the plans set so they are exclusive and one is mandatory. We give our members the option to subscribe annually, 2 years, 3 years, and life. We see now that when an annual member upgrades to a 2 year or 3 year plan, their annual membership remains active... and we need it to expire as soon as the new plan becomes active, what are we missing?