As ak said, the base fare will not change as long as you do not exceed the number of allowed free segments and continents. Also, as you are issuing the ticket in IATA area 2, you will not have to pay the US$75, if you make the changes before your first flight.

In addition to the airport taxes and fees for your new destination(s), if you reissue with BA, you may be hit with a £25 fee for their work. At the moment BA is the only one charging this extra fee and they do not seem to do it at all locations.