Hi Kelsey and welcome to the Disney Parks Moms Panel!
Your trip is going to be here before you know it! With your arrival later in the day, I can understand why you would be thinking about trying to change your park tickets. Unfortunately, Disney Parks policy is all tickets are nontransferable and nonrefundable. You might try to call the Existing Tickets hotline, (407) 566-4985, and there's a slight chance you could have some pixie dust sprinkled on the situation.
If not, all is not lost. There are so many wonderful things to experience in the evenings at Disney Parks. My first suggestion would be to experience
Toy Story Land, especially Slinky Dog Dash, after dark. Perhaps you could make a reservation at one of the restaurants in Disney's Hollywood Studios, my favorite is 50's Prime Time Café, then visit Toy Story Land and end your evening with Fantasmic or Star Wars: A Galactic Spectacular.
If that seems a little much, Disney's Animal Kingdom is beautiful in the evening.
Pandora - The World of Avatar is highlighted with bioluminescent touches that you will only see at night. Plus you could enjoy a meal at my favorite quick service, Satu'li Canteen and finish the evening with the lovely Rivers of Light show.
I would still encourage you to consider
Mickey's Not-So-Scary Halloween Party. This party is a must-do for my family. Between the incredible decorations in the park, Trick or Treat Trails, Mickey's Boo-to-You Halloween Parade, and the Hocus Pocus Spelltacular, there is so much to see and do.
Hoping for the best and wishing you a magical vacation,
Amy H.