Hey there Louise,
Thanks for stopping by planDisney with your question - we're so glad to welcome you here!
I'm happy to report that you can upgrade
theme park tickets in almost all circumstances! I say "almost" because there are some nuanced upgrade scenarios where it might not work, for example, the new ticket would need to be equal or greater in value than the current ticket.
In general though, if you had a Tier 3 ticket and wanted to upgrade it to a
Tier 6 ticket, as an example, then you would want to
call a Cast Member at the Tickets line to discuss your upgrade options. When I've been in similar situations, they have been incredibly helpful and creative in coming up with solutions that worked well to fit within the specifics of the trips I've needed to adjust.
While I'm sure the Cast Member you speak to will also mention this, I would note that, in addition to valid theme park tickets, your friend will also need theme park reservations for the date of your visit. It looks like you all are planning on visiting next week (WOOHOO!!), so I would also recommend checking the
Theme Park Reservation Calendar to make sure there is availability for your starting park.
Currently, it looks like there are only reservations for Disneyland Park on October 12, so if you had selected Disney California Adventure Park then know that your friend will not be able to select the same starting park as you. If you both have the Park Hopper option, you could change your theme park reservation to Disneyland Park with your friend (through the My Plans section of the Disneyland Resort website) to start there in the morning and then hop over to Disney California Adventure Park as early as 11:00 AM - based on availability and park capacity.
However, if you selected Disneyland Park for your theme park reservation then you should be all set!
I hope that helps, Louise! Thanks again for your question and have a fantastic trip.
Have a Magical Day,
Greg