Hi there, Francesco, and welcome to planDisney!
Currently, to enter either theme park of the Disneyland Resort, you need both a valid
theme park ticket and
theme park reservations for each day you plan to visit. It sounds like you're planning a one-day visit and might already have your single-day ticket. If you purchased them on the Disneyland.com website, you also likely already made your theme park reservation.
If your plans have changed, you are able to change the theme park reservation date of your single-day visit, but there are a couple of additional details to note before doing so. First, the single-day theme park tickets currently use a tier system - tickets are different prices depending on the day you'd like to visit. This means that there might be a cost difference for your newly selected date. I'd recommend
calling a Cast Member at the Tickets line so they can walk you through the specific details of your situation and confirm any additional steps you might need to take.
Second, part of the process to change your theme park reservation includes canceling your current date, so you'll want to check the
Theme Park Reservation Availability Calendar for the new date you're looking at to ensure it's available!
As for your second question, the system you're looking for is now called
Disney Genie+ and includes extra perks like
unlimited digital PhotoPass downloads. This service can be purchased when you initially buy your theme park tickets, but if you've already done that then you'll be able to purchase it when you first scan your tickets to enter the park. This service is a must-buy for my family and me when we visit, because it helps us experience more attractions during our visit, so I highly recommend it. Especially if you're only visiting for one day!
Thank you again for your question, Francesco. Please feel free to come back and see us here at planDisney if you think of any follow-ups that we can help with!
Have a Magical Day,
Greg