Hello there, Leah! Welcome to planDisney. We are thrilled you've dropped by with questions for us.
Oh boy! Your visit to Disneyland Resort is coming up quickly, so let's get your answers. I am excited to hear that you are thinking about Park Hopper tickets. It sounds like you have multi-day one park per day tickets and want to upgrade just one day of these tickets to Park Hoppers. Unfortunately, you cannot upgrade just one day of a multi-day ticket to a Park Hopper. The good news is that the price for upgrading to a Park Hopper is the same price for one day or multiple days because it's one flat additional cost to add Park Hopper.
You are correct that upgrades are done at the Main Entrance Ticket Booth and that if you are coming from out of town and cannot upgrade until arrival, you risk the chance of reservations being full because you would need to make new theme park ticket reservations for your new tickets. I recommend reaching out to Ticket Services to ask any questions you have about this process and a Cast Member will be able to assist you.
Be sure to check out
Disney Parks Blog Foodie Guide and the
Halloween Time page to make sure you know everything exciting that will be happening while you are at Disneyland Resort.
Leah, I hope this helps you figure out your plans for your visit. See you real soon!
Carrie