Hey there, Courtney, and welcome to planDisney!
I can most definitely empathize with that feeling of regret! This just happened to me with a family trip I planned last year, so I just went through this upgrade process. With the
Park Hopper option, you should still be able to add that on before your trip, but you'll need to
call the Tickets line and speak with a Cast Member.
This is what I did last year, and they were able to add that Park Hopper option to our tickets. However, it could depend on the type of tickets you have and/or where you purchased them from so there is a possibility that they might need you to visit a ticket booth on the day of your arrival - depending on the details of your specific ticket situation. Again, I would reach out to one of the friendly Cast Members at the Tickets line I linked above.
I am a huge proponent of the Park Hopper option, though, and highly recommend it, especially for out-of-town Guests. My family and I are in Arizona so we typically drive to the
Disneyland Resort and spend a few days there once or twice per year. Since we're not able to visit as frequently, we love the flexibility that the Park Hopper option provides during our trip!
You'll still need to make a
theme park reservation for each day of your trip, which will be your starting park for the day. Then, starting at 11:00 AM, you'll be able to hop between both parks as you'd like. That way you can enjoy a delicious lunch at the
Lamplight Lounge in
Disney's California Adventure Park and then hop back over to
Disneyland Park for a ride through the
Haunted Mansion, which should have its amazing
holiday overlay when you're visiting in September - just in time for the
Halloween season!
Thanks again for your question, Courtney! If you think of any follow-ups, make sure to come back and see us here at planDisney. We'd love to help!
Have a Magical Day,
Greg