How reinstate the PayPal payment for my ShieldPRO subscription

Since this is PayPal, and not a credit card-based subscription, the ability to update the payment method is not available.
What you will need to do is:
  1. Cancel the subscriptions
  2. Wait for them to expire (your license will have an extra 24hrs available so your site wont lose its Pro status)
  3. Manually renew the subscription (from your Subscriptions page) and reinstate the PayPal payment by completing the Checkout on the subscription.
PayPal unfortunately is limited in flexibility, and this is the only way to current make these changes.
If you pay by credit card you wont have the same restrictions.
See here how to update your credit card payment.