Some more info on this...
In some deployments, 802.11ac APs will be rolled out in phases which will require mixing 802.11n and 802.11ac APs. Few things to consider are, as most clients move through the network, they expect to see the same channel width and modulation types in use. Roaming from an 802.11ac 80 MHz channel to an 802.11n 40 MHz or 802.11a 20 MHz channel causes some devices to become “stuck” to the higher speed AP. In some cases, devices disconnect themselves from the network, which requires manual intervention by the user. When you must take a phased approach to 802.11n deployment, Aruba recommends that you completely upgrade one floor or building at a time with new APs. In the case of mixing 11ac with legacy technologies, it is always advisable to set the channel width to the lowest common denominator i.e. fallback to 40 MHz VHT operation when 11ac has to co-exist with 11n and so on. This approach gives devices in that area the best chance of remaining connected to the network and provides a better user experience without any interoperability issues.