in K, DD's teacher noted that inflexibility was a major issue, and suggested we change up little things at home. Her first suggestion was to have everyone sit in different spots at the dinner table. Well, to our surprise, DD was so upset by the new seating arrangement that she refused to sit at the table at all and ate by herself in the kitchen.
LOL but feeling your pain.
We have a super-inflexible child, and we practice sabotage. We run out of the favorite brand of bread. The favorite shirt is sometimes in the laundry. I cook non-favorite dinners and everyone has to say something nice about them. I drive a new way to the store or (horrors) don't say where we're going ahead of time. We started by changing up one small thing at a time and went from there.
Sabotage makes it feel worse in the beginning (you may see more meltdowns and protests) but after a while the child realizes it's not the end of the world for things to be different, and they get more relaxed about change in general. It's very effective.