Well, it'll taste different, due to the brown sugar--but that depends on how much you put in. It won't be bad, but you'll probably notice a little molasses flavor if you added a lot.
Now, I gotta say that I've made several Cooper's kits and just added 1 lb. of DME, because I was trying to brew something fairly light in alcohol. They do come out fine. You probably would have been satisfied with just the 1 lb, unless you really wanted the higher alcohol.