Originally Posted by Andriyko

If your fare permits changes then I do not see why you would not be able to change the one-way flight into a return. BA defines a change as a routing change as well. After all, BA does reprice return tickets into one-ways. I would call BA and ask if they could re-price the ticket for you into a return/multi-city. Again, I don't see why that would not be allowed. You will have to pay the change fee and any additional fare. I would call BA as you can't do a change like that online.