I currently have a Kenwood head unit in my car, with a Kenwood 10-disc changer in the trunk. Is there any chance that the Pioneer AVIC-N3 will work with a Kenwood brand CD changer, or do I need to look at Pioneer's changers?

Nope, it won't control any other brand of changer. Pioneer uses the IPBUS to talk to it's other units, and Kenwood doesn't have this connection, so you couldn't control it. Now if your Kenwood changer was one of the add-on units that connects with an external display and uses the Antenna lead to pull in the sound, then yes you could hook it up and use the external display. But if you wanted to control it with the headunit, then you will need a Pinoeer brand changed, and ebay is a great place to pick one up, they are generally $50-75 less than stores.