You can purchase a new subscription that will start when your existing plan ends – so if you’re eligible for an unmissable deal on a PlayStation Plus membership, you can take advantage of the special price and keep your membership active for longer. The new membership period will begin as soon as your current plan ends.
To upgrade your membership plan to PlayStation Plus Extra or Premium, you just need to purchase a membership at that level. You will pay a pro-rated fee to bring your payments to the level of your new plan for the remainder of the paid membership period.
To change from your current plan to a lower tier plan, sign-in to the account settings page for your account for PlayStation Network and choose the Subscriptions tab to manage your PlayStation Plus membership plan. Your membership will continue on a lower tier plan after the next payment date.