I did just this when starting my RTW and purchased CDG-LHR-JNB-LHR-CDG in F as it was something like US$2,000 cheaper than ex-LHR.

I ended flying DFW-LGW-CDG (DFW-CDG was full in F on AA) as I had NO CLUE one could throwaway the originating coupon and still continue the trip. Otherwise I would've just gone from Gatwick to Heathrow and saved myself the trouble of going to Paris! Here in the states they cancel the itinerary if one doesn't fly all legs ticketed or make alternate arrangements with the airline.