Ex is being difficult about summer vacation schedule again

So we have the kids 2 weeks in July and she was supposed to get them the first week of August but now she's saying she booked a trip and needs me to take them those weeks instead. I already have plans with my girlfriend and her family. This is like the third time she's changed things last minute. Do I push back or just go with it? I don't want to fight but I'm tired of rearranging my life.