Before you have actually done A, since it might fail because of ~P (which is what the thing you said actually means), your confidence is still the same as before you came up with the plan. We’re still at t=0. Information about your plan succeeding or not hasn’t arrived yet.
Now if over the course of planning you realize that the very ability you have to make the plan shifts probability estimate of P, then we’ve already got the new evidence. We’re at t=1, and the probability has shifted rightfully without violating the law. The evidence is no longer expected, it’s already here!
Before you started planning, you didn’t know that you would succeed and get this information. Not for certain. Or if you did, your estimate of probability of P was clearly wrong, but you hadn’t noticed it yet, where the “yet” is the time factor that distinguishes between t0 and t1 again...
Can’t cheat your way out of this at t=0, I’m afraid.