Removing cabinets and matching Ceiling knockdown underneath
One way to reduce the visible ridge effect is to use sandpaper to taper down the old knock down material along the ridge to smooth it out some. This job will be a bit harder if the old material has coats of paint on it. When I have sanded like this in a similar situation I used 120 weight sandpaper on a hand sanding block.

Note that there will be the urging desire to perform this sanding with a power sander but this is generally not recommended because it is so easy to sand too far and end up tearing up the paper covering of the drywall.