Join Date: Dec 2008
Location: Mostly living in the basement
Programs: Free Agent Level 2; MR LT(!)TE, HH SE, ALL SE, BAC Silver, DL MM, UA PS, 2V Fanboi, CBP GE
Posts: 5,359
AIUI you can't change the FF# after the itinerary is started, but perhaps others have experience doing so.
In retrospect, booking two one-ways would solve this problem.