The AlphaGo team has stated that they no longer have the ability to make changes to the AI used for the match. The rules of the exhibition match prohibit that.
They maybe could have anyway, but it would be cheating: just the same as if they'd Mechanical Turk'ed it by e.g. having Ke Jie actually choose the moves to play.
They maybe could have anyway, but it would be cheating: just the same as if they'd Mechanical Turk'ed it by e.g. having Ke Jie actually choose the moves to play.