Okay, this is obviously the issue then. Voila! Problem solved.

On EF, you can't see the ex-KTM POS! The USA is set to the default POS, although you can change to many other cities and countries. Unfortunately for you, Bangladesh / DAC is not one that EF provides access to.

The fact you can see I class ex-JFK is irrelevant. Since that's displaying only I fare for the relevant POS you have selected. Normally, this isn't too big a deal, as places like HK and the USA tend to have similar seats available per fare bucket for each POS.

But some of the outports, especially cheaper ones do often have significantly less sub fare buckets available. I've never flown to DAC before but I imagine ex-DAC tickets are on the cheaper side.

Either way, this is definitely the problem. The call rep didn't precisely explain the problem, although what they said technically isn't wrong. I fare isn't available (unsaid: from your POS) on the date you want to change to. Even if there is I fare available for the ex-USA, ex-HKG, etc. POS for the flight you want, you have to wait until it's released for ex-DAC inventory.