Hi Bargainhunter,
Your question is something that a lot of astrology enthuasiasts may have thought about it before.
From a logical standpoint, I think it works within a particular boundary. It is a bit like a Fengshui master giving advice to an adult and he avoided a fatal car accident because of that. If that adult has potential to be saved, he will bump into this Fengshui master. If the adult has no potential to be saved, he may instead bump into a quack Fengshui master.
But a Fengshui master is no miracle healer. If the change is too drastic or not within the allowable boundaries of that person's destiny, there is no way the Fengshui master can change the outcome. For example, the Fengshui master could have selected a good date and time for the C-section but unfortunately, either the doctor gets into accident on way to hospital for the surgery or complication of the mother's or baby's condition arise that they have to take out the baby earlier.
As to whether someone have to pay the price for changing the course of destiny, this is something no master dare guarantee. Although one of the conventional argument is - if helping someone avoid death is changing destiny and punishable, then should not doctors that cure those terminally ill ones be also punishable?
I would think that even having the chance for a c-section is the result of good karma, be it the efforts of the ancestor, parents or who ever you can think of. Without good karma, one might even 'select' the 'wrong' date for a c-section.