I do not think you can change the tickets after they are purchased....however you can apply tickets you currently hold to new tickets....so I would assume you could always just apply the value of the hopper tickets to new no-expire tickets (since they would cost more) and just pay the difference.
This is really more of a bump than a definitive answer for the OP,,,,I do know they will give you credit for the face value of a ticket you hold and apply it to new tickets....we were able to exchange some MYW tickets for AP's and while we were doing it the woman at the window next to us was exchanging her new 4 day MYW ticket with no options for a longer ticket (7 days) with plus options and parkhopping....just not sure if they would offer a refund if you selected a cheaper option (but my best guess is probably not)