Your dealer is a boob.
But then, asking for the correct plug in the first place would have resolved the problem before it happened.
J-who is correct, the oem spec plug for your bike is a BR8ES. If your bike has not been jetted properly (meaning changed from the 48-160-R11 mess that it came with) you're going to have quite a few problems with the oem plug, let alone a one-step colder one.
At the very least, run the spec'd plug. Preferably a fine-wire iteration of the same heat range..say a BR8EG. Jet your bike correctly via the scillion (or so)threads on this board that have information about that.
Or..(again) at the very least.....
Go to the site you bookmarked with the link on the post stuck to the top of this board (every kdx rider should read...) and read what CDave says about jetting related to the modification level of your bike.
Your bike will run much better. You will no longer foul plugs at all. Changing your plug may well turn into an annual 'tune-up' event.
Take those #9s back. Trade 'em in for BR8EGs. Tell the dealer, 'Thanks...boob!'