Hi,
well it depend,
the migration tool will help you to migrate your configuration from 6.5 to version 8. It will give you 2 different configuration migreated. Default-Local and default Global..... default-local is proper to the controller, and default-global is like an global configuration you will pushed to all your controllers.....
i recommend to do an clean-up after using the migration tool because you will have a lot of default configuration.
On my last deployment, my client worked with an hybrid environment, mobility master with 1 controllers (version 8) and also 1 master with 2 local controllers (version 6.5)