Ex wants to change custody schedule last minute again

This is the third time in two months she's asking me to swap weekends. I get it, life happens, but I've already told the kids we're doing stuff this weekend and made plans. How do you guys handle this without being the bad guy? I don't want to be difficult but I also can't keep rearranging my entire life. My lawyer said document everything but that feels petty when it's about the kids.