Last minute changes are part and parcel of a creative work environment. I listen and accommodate the changes. If there are major consequences, it's expected from me to outline them and bring them to the client's attention, so alternative ways to fulfil the brief will be found with the client's knowledge and consent.

If it’s something you can reasonably handle, just do it. If not, communicate that with them. Photography contracts should specify a deadline for specific requests for coverage to ensure that you have the proper staff and equipment to deliver high quality images to your client. Certain requests may require an update to your contract to ensure you are being properly compensated for the work you are doing for them.

To prevent some of the last minute-requests and to even give yourself something to motivate yourself when these kind of requests happen, don't forget: this is a special service feature - it should be compensated adequately. Inform customers about your prices in advance. Of course you can go "Full Service = One Price" if you want to, but then prepare yourself for occasional night shifts to fix problems, you did not cause, that will ruin your mood and your sleep schedule. You may even start to hate some of your customers because you feel they are taking advantage of your best intentions if it happens more often or your extra work goes unnoticed.

These days clients expect almost immediate delivery. In saying that, you need to communicate these expectations prior to the shoot, in writing. If they expect changes then I try to help but sometimes it's just not possible. Try to anticipate client needs also in this case, keep communicating with them during the day so you don't get any last minute surprises!