Ah, the joys of kittens! I keep my cat food in a big plastic container and often keep the new bag in the back of the car until I really need to open it. Do you have a jar or some other kind of dish you can put the old stuff in to use up once you put the new bag in the big cannister or whatever? Or just dump it out on a plate and put it on top of the new stuff so it gets used up first.