I did not find that "change my flight button" today in th KL App neither yesterday, nor today before nor today after i checked-in.
Booking was done through a third party, should not make a difference, I changed such a flight already before through the TA, but maybe it does...
maybe because it is now the last flight of the day, no more earlier flights, but there would be another one within the 14 hour window.
anyway, i dont intend to change the flight also checked in a bag already, i was just wondering for the next time..