A customer can only have one subscription covering a given period. If they try to buy a plan whose start date is on or before their current subscription's last scheduled delivery date, checkout refuses with "You are trying to create a subscription on a date you already have an active plan." (with a hint to create a membership for another person when account members are available). Separately, when Client Config > Order: Subscription > Active Subscription Warning Popup is Yes, a customer who already has an auto-renewing subscription sees a warning modal on the plan chooser telling them when it renews. To let them switch plans, pause or cancel the current subscription (or let its dates run out) before they buy the new one. In the current version, the old subscription isn't disabled automatically at the moment of purchase – it's the renewal cron that, on the old subscription's own next renewal date, notices a newer subscription already exists and turns off its auto-renew, after which it shows on reports as "Paused" (system).