#1 rule is to keep the channel width at 40. In addition, try to upgrade a whole floor or building at a time to 11ac.
As per the standard 11ac is backwards compatible with 11n. 11ac and 11n APs can co-exist in the same network, similarly 11ac and 11n clients can co-exist in the same network as well. Aruba recommends that you completely upgrade one floor or building at a time with new APs in order to phase out the older technology with the newer one. This is a pre-cautionary step. 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 due to poor client driver implementation.