Hey Mark,
Thanks for your question and welcome to planDisney! I always love when I get to deliver good news, and it certainly sounds like I get to do so for your situation.
While your
theme park tickets from 2017 are most certainly expired, you should definitely be able to apply them as a credit toward the purchase of new theme park tickets and just pay the difference!
Having said that, there are some minor caveats to this in the
terms & conditions (e.g. your new tickets need to be equal or greater in value than your original tickets, it does not work for special event tickets, etc), but more than likely you should be all set.
I would recommend
calling the Tickets line to speak with a Cast Member about this. Kudos to you for keeping the original order number handy - they will most certainly need that - and they can help you purchase your new tickets.
Currently, you're able to book vacations to the Disneyland Resort through the end of 2023, but I would anticipate the 2024 bookings to open up here in the next month or two. Either way, your new tickets will likely be good to use until January 12, 2025 if you are purchasing multi-day tickets.
Thank you again for your question, Mark! Come back and see us if you think of other follow-ups. We'd love to help!
Have a Magical Day,
Greg