Yes. One is not allowed to change flights past security, no matter if it’s voluntary (on a flex flight) or involuntary (flight cancellation). Flight changes can only be done at the checkin counter. This has to do with the border and immigration control.

As a result, if there is any flight cancellation leading to change of flight, one has to enter China again, change the flight at checkin counter and clear customs/immigration again regardless of whether you have checked baggages. You cannot move to an earlier flight at the lounge if there’s a delay.