I have been in this situation, and honestly, I just say what I would want to hear if/when I'm struggling with the HS decision. It's obvious that you care about these friends very much, so you should start by telling them that. And making sure that they know that you know they'll do what is best for their family because they are great at being moms. Definitely tell them you will miss spending time with them, but that you support their choice whatever it may be.
One thing I was so surprised about when I had my first child was how hard it was to make friends with other moms. It really is like the dating scene all over again...at least it was for me. None of my friends had children, and I stayed at home with my baby 30 miles from where I had been living until I gave birth. I had no one, and really missed the support of my friends. As a few years past and life changed in lots of ways, I got over feeling isolated and alone. It hasn't been until now that I finally feel like I am meeting moms I really love. Because of our choice to HS, I am meeting AMAZING people all the time. People I have lots in common with and that I can actually like on a genuine friendship level because we have so much more in common than just having kids. So, I guess what I'm saying, is that even if you see less of those friends, I am confident that you will find lots of other wonderful people to be with in the course of your HS adventures. It's such a great community! Good luck.