A same-night change, and the moment she sets out
A same-night change is a later or earlier hour, or a cancellation, asked for on the evening itself; whether she has set out decides what it counts as.
An evening out rarely keeps to plan, so changes on the night are ordinary. What each one counts as depends almost entirely on a single fact: whether she has already set out.
A later hour, before she leaves
While she has not yet left, a later start on the same night is an adjustment to the hour you agreed, not a cancellation and not a move to another date. It costs nothing when she can keep the new time. If she cannot, the reply says so and tells you who is free at the later hour, and you choose whether to name one of them, keep the original hour, or cancel. A cancellation at that point is read on the ordinary notice scale.
An earlier hour
Ask. If she can set out sooner, the reply says so; if not, the original hour stands. Asking costs nothing.
A change after she has set out
Once she is on her way, her journey has begun and part of the fee is already earned. Write at once with an honest time. What can be done depends on her and the rest of her night. If it cannot go ahead, it counts as a cancellation sent once she was already on her way.
Moving to another night on the night
A move to a different date requested inside the final twenty-four hours is measured as a cancellation sent at that moment would be, and whatever the scale does not keep travels on to the new date. The free change of date needs more than a day's notice and a new date within fourteen days.
The guide to a night that runs long works through each case with figures.