Thanks for your great question, Katie.
It seems like you are interested in a double-dose of Disneyland Resort magic. What a great idea.
Making changes and upgrades to Disneyland Resort park tickets can be done at the Main Entrance ticket booths. I learned from a cast member that changing a one-day Park Hopper ticket to a two-day ticket for admission to one park per day should be allowed. Prices and terms are subject to change, and are available by stopping by the ticket booths. Note that a conversion from Park Hopper ticket to non-Park Hopper ticket would require that the ticket be completely unused before converting it at the ticket booth. Also, one-day
tickets are priced differently for Value, Regular and Peak days. So the price difference to a two-day ticket varies accordingly.
Once your ticket is converted, you are off for two days of adventure. I recommend you experience the newly re-imagined
Pixar Pier in Disney California Adventure Park and the
upcoming
Get Your Ears On – A Mickey and Minnie Celebration beginning later this week.
Have a magical time!
~Tony