Yes, I have done this succesfully in my lab with 1x local (but I'm not sure if it was 8.4.0.5 exaclty)
I suggest you do this in a lab device first before trying it in remote locations.
Some lessons I learned:
- Don't play with the same controller going up and down multiple times between 6 to 8. The database doesn't like this.
- If everything goes bad with the migration tool and you need to just force an upgrade to 8.x, your config on the remote device will be lost and you will need to run the startup wizard again, so have a local user with console cable on standby. And be prepared to rebuild your configuration from scratch.