Hi, Brent!
Unless you have added the PARK HOPPER option to your tickets, you
cannot enter 2 parks on the same day. Without the Park Hopper option, you have "base" tickets which is one park per day. The turnstiles are very smart, and they will read your ticket and will know you already used your ticket in another park and will deny you entry. You are welcome to exit and return to the same park, but cannot switch without that hopper option.
If you wish to park hop, simply add the Park Hopper option to your tickets, about $55 for your entire stay. I don't know your package, arrival dates, or any details, but you might be able adjust your package and remove the day you will not be using. Call (407)W-DISNEY to see if you want to adjust your tickets.
And your last option: add the NO EXPIRATION option to your tickets and you will have the extra ticket for your next visit. This option can get pricey, so weigh your options on this one. Upgrading your tickets to Park Hopper or No Expiration can be done at any ticket window.
I know this is confusing; I hope it all works out for you! If you need more clarification, please
let me know!