How do I know when to change the plan mid-operation?

You can follow the plan well. What eludes you is the harder judgment: knowing when the plan is wrong and it is time to change course mid-operation. That is one of the deepest skills in your work, and it is supposed to feel hard. Following is about discipline. Deviating is about reading reality faster than your plan can. The plan is a guess made before you saw what is actually in front of you. Good judgment means holding the plan loosely enough to notice when what you are seeing no longer matches what you expected. The signal to change is usually that quiet mismatch, the thing that does not fit, that you are tempted to explain away. So train yourself to name it out loud when something does not match the plan, even to yourself: this is not what I expected. Do not rush past it. That pause, that noticing, is where the decision to change course lives. You already know how to follow. Learning to question the plan is just learning to trust what your eyes are telling you over what the plan told you to expect.