Hi Lindsey!
Welcome to the Disney Parks Moms Panel, and thank you so much for bringing your Disneyland Resort theme park ticket question to us hEAR!
This is a great question, because no matter the answer you'll still get to have a Disney Day! I sadly also do not have a definitive answer as this decision depends on the kind of admission ticket you have and can only be made once in-person at the ticket booth. If the ticket in question is a true 'complimentary pass' that you might have won in a contest (woohoo, lucky you!), is a media ticket (tag us online at #DisneyMP!), received as a make-up due to another park day cancellation, etc, then there should be verbiage on the ticket itself about what it can and cannot do, like "not to be used toward payment of an Annual Pass".
However, if your ticket is none of the above, or if this admission is one that someone purchased and gifted to you, then there's definitely hope. I was in a sort of similar situation last summer where I initially purchased 1-day, 1-park tickets from a military base weeks in advance, and day-of was able to successfully switch to a new park hopper promotional ticket that had been introduced a few days prior by paying the difference between the two. I feel strongly that this will be your option.
I'm not sure when you are hoping to visit, but due to the
temporary closure of Disneyland Resort new ticket sales are currently paused, which includes being able to see ticket prices online. Prior to the close the difference between a highest tier 1-Day Park Hopper, which can be used any day of the year, and a 2-day 1-park-per-day ticket was less than *$50, so that's a good ballpark number to use for budgeting purposes. (*Do note that prices are subject to change without notice.)
Bonus Tip: To stay on top of everything relating to the reopening of The Happiest Place on Earth, go ahead and upload the ticket that you do have to the
Disneyland Mobile App, as well as 'like' and 'follow' the
Disney Parks Blog across all of your social media channels to be sure to get the newest news as it's announced.
I'll wish for extra pixie dust for your upcoming visit, Lindsey, and if you have any more Disneyland Resort planning questions, We're All Ears!
~Kristen