Thanks to all. I have a four month break in Boston between the first ten segments (asia and SWP) and the next ten segments that include the destination change. I have a four hour layover at LAX on the return from SWP. I suspect either the BA or AA ticket office at LAX can do the job.

Much easier than having IST make the change!!

ps. I booked it well in advance to make sure I got the flights I wanted and to hedge a fuel increase. I booked it ahead of the fuel surcharges.