There's some airlines that advertise "no change fees" for all their tickets including economy, like Qatar and Emirates. Make sure you book directly from the airline, the ticket may be slightly more expensive, but well worth it if you need to change.
Example:
Qatar
Heathrow - Phuket:
£510 from agency
£564 direct with airline
From Qatar's website:
Double check the "no change fees" policy is still in force before you book your ticket ????