Welcome to planDisney, Harry! I'm sorry to hear that one of the members of your travel party is sick. We get similar concerns from many guests, so I'm happy to provide you with some options.
I chatted with a Cast Member today, and she confirmed that as long as the tickets are completely unused, they can be gifted to someone else; therefore, while it is recommended that you change the name, it's not necessary. As long as that person presents a valid park ticket and park reservation, they can enter. Once the ticket is scanned at the gate entrance is when it will get assigned to that guest.
If you decide you'd still like to change the name on your theme park tickets, you'll need to contact
Disneyland Ticketing. Since you are an international guest, I realize that calling may be difficult. You can try doing it via the
Disneyland Mobile app, where you can directly message a Cast Member for customer support. To do this, open the Disneyland app > menu > click "Chat With Us." While a name change is not guaranteed, the Cast Member will often send you a form for a name correction. It's possible that the app may not function for you until you reach the United States. In that case, you can wait until you arrive. You can also visit any theme park Main Entrance Ticket Booth, and they can assist you; in my experience, the ticket booths open an hour before the advertised theme park opening time, so plan to arrive a little early on your first park day.
You'll be visiting at the perfect time to enjoy the return of the
Disneyland Resort nighttime spectaculars! My favorites are the
Main Street Electrical Parade, which will have a new grand finale, and
World of Color. Try to make some time in your schedule for at least one of these!
I hope this helps, Harry! Good luck, and we'll see ya real soon!
Eean