A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Bayushi Arashii reads...
Scorpion Clan • Samurai • Ninja
Battle: Ranged 3 Attack. If the Ranged Attack targeted a Personality, you may compare it against his Chi instead of Force.
My opponent wants to redirect it to either Shiba Yukihito or Shiba Raiden. When do I have to declare that the RA is against force or chi?
Bayushi Arashii: When do I have to declare that the RA is against force or chi?
At the point you compare the strength versus the stat.
Targeting is referred to in the past tense -- which is when most effects that change targets occur, natch -- so it's clearly after that key point.