I booked a flight number 8 from Harbin to Nantong, but it was cancelled later and I changed it to the 11th.

According to the airline’s change policy, if you have changed your ticket to the 11th, usually you need to cancel the original changed ticket on the 11th, and then apply to change the ticket to the Harbin to Yancheng flight on the 8th. Whether the ticket can be changed and whether additional handling fees need to be paid depends on the specific regulations of the airline. It is recommended that you contact the airline customer service directly to learn the detailed change process and fees.