Ex keeps changing pickup times last minute, killing my schedule

This has been happening for 3 weeks now. I'll plan my work around our custody schedule and then she texts me an hour before pickup saying she needs to switch days. I work construction and can't just leave a job site whenever. I've tried being nice about it but it's getting ridiculous. Do I need to get the custody agreement updated to be more strict about this? We agreed to be flexible for the kids but this feels like she's taking advantage of it.