Hiya, Tiffany! Thanks for bringing your question to planDisney!
I'm glad that you are looking into purchasing the
Park Hopper option! With Disneyland Park and Disney California Adventure Park being so close to each other, park hopping is an absolute breeze. My family and I park hop every day of each vacation we take as it allows for maximum flexibility in our park touring strategy.
You can absolutely upgrade your one-park per day ticket to a Park Hopper ticket, but this must be done in person at a Disneyland Resort ticket booth. Keep in mind, though, that the cost to add the Park Hopper option is per ticket, not per day. Meaning that it will be the same flat price to add the Park Hopper option for all three days as it is to only upgrade to that option on your last day. So, it would make the most financial sense to add it from the get-go if you are planning to hop at all during your visit.
You will also need to
make theme park reservations for each day that you visit. Theme park reservations can be made up to 120 days in advance. With a Park Hopper ticket, you will make reservations for the park you want to start at each day. Then at 1:00 p.m., you can hop to the other park and continue hopping back and forth the rest of the day if you wish (subject to park capacity limits).
If you have any other questions as you plan your adventure to the Happiest Place on Earth, please stop by planDisney again!
-Linda