Thanks for all your replies. I don't want to get too much into specifics here..I may have found a solution and will advise here if so. Essentially the problem is as follows;
I have booked to go to AAA, but i want to come back from BBB. Example fares as follows;
LHR/AAA - A1 - 5000
LHR/BBB - A1 - 4500
LHR/BBB - A2 - 6500
So I am told that coming back from BBB i can only be changed to the A2 fare as the A1 fare is lower than the A1 fare of AAA!
The problem is that this is a TA fare, not published and therefore it makes it more complex!!!