Hello Patrice!
Welcome to planDisney and thanks for bringing your question our way!
The mistake you’re describing actually happens quite often. Please don't worry! You will be able to make this upgrade and enjoy your visit. But, to upgrade a child’s ticket to an adult
ticket, you will have to make this change in person at the
Disneyland Resort Main Entrance Ticket Booth. A Cast Member will be happy to assist you. Please be prepared to pay the price difference in full. Also, I am happy to hear that you’ve already made your Advanced Theme Park Reservation. You’ll need that reservation in addition to your valid theme park admission in order to gain entry. Your
Theme Park Reservation should carry over once the update is made.
Enjoy your trip in February! If you have any more questions, bring them to us here at planDisney!
Tiffanie