There are several thoughts on this and it all is site specific really. Can you share more about what this upgrade entails and use cases/business requirements?
Basically, you CAN mix and match although the channel widths should be set to 40 MHz vs a mix of 80 and 40. This is to avoid any client confusion and channel overlaps in the overall RF network. We recommend that an entire wing/floor be swapped out if budgets don't allow for a complete building upgrade. However, a salt and pepper can work if configured appropriately although used as a last resort.
If you are doing a 1 for 1 replacement, is the current deployment based on coverage or density? What is the signal quality today and will new applications like voice and video be supported by the new deployemnt? This will dictate if more APs may be needed. In some cases, we are doing a 1 for 1 and in others, we are adding to support more density or improving the signal quality.
The ARM behavior should be fine and not need to be tweaked epecially with ClientMatch on 6.3.
See here for more depth - http://www.arubanetworks.com/pdf/technology/MG_80211ac.pdf