I have done something like this, but it was over 13 years ago. I had a ticket SFO-LHR-HAM (visiting family) and then work came along and wanted me to stop on the way back in London for several days.

Original I was flying back on a Sunday. Called up BA to move my return flight to Friday following that Sunday, they added a note/protect to it and then bought a new one-way for HAM-LHR on Sunday.

Wasn't a problem back then, but who knows how they handle this today.