Join Date: Jul 2007
Location: San Francisco/Sydney
Programs: UA 1K/MM, Hilton Diamond, Marriott Something, IHG Gold, Hertz PC, Avis PC
Posts: 8,164
Odds are you won't be able to change it online - or at least not on the United website. You used to be able to, but they removed the ability a year or two back.
You may be able to change your FF number to the United one on the Krisflyer website, or worst case United can do it over the phone as long as you make the change before check-in for the first flight.