Yes, Koelsch can be very different between the breweries. Altbier can also be very different from place to place in Duesseldorf.
Herr Strong may have something to add.
OK, I guess you wanted something more? Yes, they were all quite different. I was amazed at how different Kölsch was from place to place. There were a few attributes that people would change, but they would only vary by small amounts. However, those changes would make a completely different beer. I took very detailed notes because I was trying to bound those various attributes to put into the 2008 BJCP guidelines. So if you look at things like hop aroma, hop flavor, esters, malt flavor, body, dryness, etc., you'll see ranges. That's what I learned from going around to all the various breweries in Köln. Same thing in Düsseldorf. Same trip. The interesting thing there was that most breweries had small variations like in Köln, except for Zum Uerige. They were way different. The older (pre-2004) BJCP guidelines were written for that one beer, but most examples there aren't like it. It's an amazing beer, but it's sort of like the Fuller's ESB of Düsseldorf.
So basically what I learned in those two cities shows up in the 2008 BJCP guidelines. If you see the changes between 2004 and 2008, the difference was that I went there to validate what was actually produced at the source.