Sure, a structured parenting schedule (i.e., note that kids don't VISIT parents) would be fine.
Work with your ex to determine the schedule that would be best for the kids.
The part that you're going to hate is that the kids won't be with you as much as the past many years.
It's very understandable that you'd dislike that (what parent would want to give up any time with their kids?).
However, you have to recognize that their dad moved to be only 2 miles away from you for a specific purpose. He wants to be more involved, obviously.
It's reasonable for you to want to keep their routine. So, perhaps your first communication with your ex is, "Ex, now that you're closer, I think we need to talk about a new arrangement for when the kids are with each of us. I just want to share that my biggest concern is for the kids to keep their routine... so maybe we can figure out how to do that together."
Teenagers can easily keep routines in two homes (e.g., bedtimes, homework, going to sports practice, going to social clubs). So, you and your ex just need to be on the same page with what all that means.
The POSITIVE of this all for YOU is that you can expect to see your kids much more in the summer now.
It really is up to you how you approach this-- clearly, your ex moved closer to be near the kids. It's his goal to spend more time with him, and I bet he's going to make that happen one way or another. If you ignore him, he's left with court as the only option.
If the two of you work together, each making concessions, and figure out what's best for the kids together... then draw up a new parenting plan and file it with the court, you'll have done extremely well.
Plus, try to stop the kids from seeing their dad (if they enjoy him, and if he's not a threat to them), and your kids will eventually hate you.
I hope this all has helped offer you perhaps a different perspective.
Is it likely that he'll get sole custody of the kids anytime soon? Nope. Unless you're a drug addict or prostitute or child molestor, the only way it seems you'd ever lose custody of the kids is if you severely restrict their ability to have a father.
So, perhaps for your own long-term interest in avoiding a change of custody, give dad more time ASAP, and draw up a new agreement for the same.