OK. A 8.x Mobility Master cannot terminate any APs. AP adoption will also not work meaning that your AP discovery (Option 43 or DNS) must point to a manged device under the MM.
As for migration there are two options:
1. Do not use a mobility master (VM or HW) and use "master controller mode" which delivers the new UI and some of the new features but allows you to upgrade and not have to deploy a new appliance. NOTE that you cannot terminate APs on a controller in Master Controller Mode.
2. Use the MM and gain all the benefits and get a two-node cluster with Live Upgrade, Seamless Failovers, and User/AP load balancing. Possible expansions into DMZ/RAP controllers, Branch networks with a branch gateway, etc...are also possible.
So, yes, there is another appliance needed but there are several benefits in deploying one. Keep in mind, we have also released a MM-50 SKU for smaller/lab deployments as well.