I've got no help, but - very interesting question. I'd not considered the impact of this.
I am looking to deploy my first 802.11ac devices in the next month or two. They'll be located near a bunch of 802.11n devices, so when it's all installed I'll be able to experiment!
On the face of it, i'd think that by probing a client would be able to successfully identify suitable networks nearby and when joining a new one, configure itself according to the advertised capabilities of the new network.