There is a long explanation here:

https://www.qatarairways.com/tradepo...d-Charges.html

Amongst all that is an explanation that where segments are married together (ie. you booked a ticket for BKK-LHR, not BKK-DOH and then DOH-LHR separately), it is not possible to cancel, price or ticket a segment individually. So the agent was correct and this is probably the same on many/most airlines.

You would need a complete BKK-LHR itinerary available to book today with avios to be able to switch schedules to that itinerary. Also be aware that if you cancel hoping to immediately rebook, your current seats on BKK-DOH may not become reavailable for avios booking as the flight may be almost sold out and that space is now being held for cash buyers. Alternatively, you could cancel and try to book each segment separately if they are available separately and the full journey is not, but that will cost more.

If there are no preferable BKK-LHR redemptions available, I would be inclined to stick with my original booking