This is my experience trying to change my FFP number. I am part way through a four-sector itinerary with UA. I'd selected SQ as my FFP before the trip, as the first sector's fare class earned more miles in that programme. After that first flight, I tried to change it online to A3 for the rest of the flights (for Gold requalification). Couldn't do it. Read on FT that I could do it via the Twitter team. That worked! Checking in for the second flight two days after the first sector, it was correctly reflected on my reservation and my BP. Unfortunately, after my second sector, the first sector posted in my A3 account despite only changing to it after the flight!