Mmm, I could definitely see older clients being unhappy at being asked for stronger ciphers. In reading the patch notes, that looks like it applies to site-to-site tunnels? "The DES encryption algorithm is no longer supported for use in formation of VPN tunnels." If I recall correctly, the existing client VPN defaults to 3DES already... no DES involved.
There's also this under known issues: "client VPN users cannot communicate across non-Meraki VPN tunnels"
So uh, personally, looks like I'm really unlikely to be using 15 until the beta gets less beta-y.