File IMMEDIATELY for a modification of custody. Even if it's just an "empty threat" or a "passing fancy" - you can get a provision added to the current custody order to prevent or otherwise limit where you BOTH can go with first a hearing on the matter.
Further, you're in a good spot with being the primary residential parent -- at least in terms of indisputable issues, such as school district, which gives you a little bit of an advantage when it comes to preventing a move-away.
As a backup, I would strongly suggest you start creating a parenting plan, loosely based, on what the potential arrangement might be with you having primary physical custody in order to minimize the disruption in the children's lives and maintain the continuity. Not something you want to share, just something you want to have worked on in the event the time comes.
Be prepared. Always.