This is what I tell people from experience. If you want her to be flexible in the future when you need it, you have to pick and choose when you are going to be rigid or flexible. Is it doable for you without cutting your plans short? If not, I wouldn't because there is no emergency.
I know many say when you "give in" it's cake eating. But in the reality of a long term situation of custody schedules, it's good to be flexible when possible, but not all the time. You might need it back one day. Only you know if she would repay the favor if you needed.
