I have changes DOWE4 tickets several times and not been charged at all. When making the changes by phone I have always been told there would be a US$75 charge, but when the ticket was re-issued there was no charge.

Each time I have had the ticket reissued at LHR T4 or T1 BA ticket counter and the staff have been very helpful and efficient. When I asked about the charge, with credit card in hand, I was told there would not be a charge.

I believe you can get one change/reissue for free, but I don't know if that is restricted to tickets originating in certain places. Mine always originate from Australia.

The last one I changed by adding an LHR-FRA-LHR segment and changed the return to Australia forward a day and routed LHR-SIN-SYD-BNE rather than LHR-HKG-CNS-BNE. Even though I requested/booked the changes while in the US, I deliberately did not have the ticket reissued until LHR due to my previous good experience there.