A plan is a direction, not a contract. Its job is to keep you from drifting, and it does that even when most of the details turn out wrong. People confuse having a plan with being obliged to follow it, so they treat new information as a threat. Year two usually arrives carrying facts you could not have known when you wrote the thing, and refusing them in order to stay consistent is not discipline.

Write the plan, then put a review date on it, maybe your birthday. When something unexpected shows up, ask whether it moves you the same direction by a different road. Often enough it does.