Far as i know You need a configuration migration because 6.x isnt the same architecture as 8.x.
Couple of weeks ago i pre-install a new 6.x controller (only give a ip etc.) then update to 8.x and the controller was empty again. I think that that part is where the migrationtool kicks in to upload the converted configuration.
Personaly i only have migrate manualy (by rebuild the configuration) so far, no real experience with the migrationtool.
Hope this help you in the right direction.