hook to verify client's info on Upgrade/Downgrade a package, before payment process
Add a hook that works when existing clients
Upgrade/Downgrade a package or Upgrade/Downgrade Options on client's
area. We can verify client's info before they proceed to pay the Upgrade/Downgrade order.
Basically, it is another version of hook ShoppingCartValidateCheckout. This new hook is run and an error can be returned to stop the Upgrade/Downgrade order process, and show a message to client.