While the BK-7m has a key range parameter for each live part, this feature is missing in the BK-9. This means you can only split lower/upper by using the SPLIT button.
Why is this important (for me)?
Because in the BK-7m I could use 4 live parts flexibly; the "Manual Bass" part could be used like "Lower" if you didn't use the dedicated split function, but instead the key range setting. You could layer a piano plus strings left, and still have two upper parts. Not possible with the BK-9. The key range parameter has been dropped. I'm quite shocked.