Thanks for the replies guys.

I've found an example that shows the issue I'm talking about. Flying BKK-LHR, then FRA-BKK on Gulf Air departing Feb 9, returning Feb 19 is about $3000.

Now if I fly out on the same date but return on 15 June instead, the fare jumps to $6000.

I'm not really looking to book this, or any fare per say, but I just sort of want to understand the whole date change fiasco which really confuses me immensely.

Say the fare conditions state that date changes are allowed at a $150 fee, but tickets are nonrefundable.

So if I book the fare, and then call them to change the date of the FRA-BKK sector, I assume the realistic answer be that the fare difference will be substantial?

Would it make any difference if I flew out on the BKK-LHR leg and then called them up to change?

Also, how is the fare difference calculated? Is it the overall fare or by sector?

Thanks for the help!