Originally Posted by
sbm12
The fare for the new itinerary will be recalculated based on the fare difference between the old one and the new one. In this case it should price only changing the return half if you make the change after the outbound, with the caveat that it will depend on what inventory and fares are available at that time. And, yes, it should still be able to use the restricted, return fare rates rather than just a one-way ticket.
Did it last year for my BH (what is it with BHs not making their mind up?) in OP's exact same situation (oh, my friend's going to be in London, can you re-route me to leave through LON instead of FRA, etc etc etc).
So, I had to change LIN-FRA-SFO to LHR-SFO (and get a mileage OW ticket LIN-LHR with some handy leftover BA miles). 1K agent spent a good bit of time with me trying several combos of LHR-SFO to find something that would price so that I'd not be out much more than the 250 (as this was rather late) and we ended up with LHR-LAX-SFO which cost me something like 256 bucks, so we did pretty well. No sweat in principle, the only issue as sbm12 says is you need to get a bit lucky finding fare buckets, so I'd do what clacko says and price some itins for yourself so you can find one that will work within the financial bounds you are willing to live with, either that or you'll have to have the agent do it on the phone which can actually be more time consuming.