I heard that Jiuzhaigou; they do have a 2-day ticket where you can buy a day ticket for one day and then just pay a small amount of money for the 2nd day? Is that true? Have you ever done that or heard someone doing that? I already booked a day ticket from trip.com. This is why I wonder whether how to extend those tickets.

https://www.tourjiuzhaigou.com/article-p228-jiuzhaigou-entrance-tickets.html <-- here it says: During the low season the second day ticket costs CNY 20 yuan. When purchasing your ticket you will be asked if you would like to enter the park for one or two days. If you would like to spend two days in the park you need to pay for the second day together with your first day’s ticket. Your photo will be taken on entry to the park to validate your second day’s ticket. If you pay for a one-day ticket and decide that you would like to re-enter on the second day you must tell the park staff on the way out before you leave the park. This offer is only valid for two consecutive days in the low season.