I got confirmation from Accurate. Excellent customer service.
You only need the gear set to upgrade from 4:1 to 6:1. The parts you will need are B-5782 (main) and B-1514 (pinion). Less than $50 for both. You can order from them.
Have you got the gears and replaced them, doest it really work?
The reason why I asked actually is my own experience with the old 870. Years ago I wanted to change it to 6:1 and for that I needed to change the complete side plate where the gears are, also. That's caused by the different diameters of the 6:1 gears.
So now I'm really curious about the gear swap on this new 400 series, please post your gear change process. I also have the BX400XN 6:1, it would be nice to have a second set 4:1 gears. Thanks