It really depends... if you are changing a minor trait its one thing... but if your making some change to a core thing that defines the creature, then you should just make a new creature.
So what is the core of a zombie? Well im not an expert but i would guess they are dead and rotting, and want to eat people. I would think dumb is probably also core, but what if yours get dumber with time? I.e. they start off with some intelligence, and then it fades but they can slow down the fade by eating brains? I dunno... not a zombie expert.

But I think the principle applies, as long as what ever is the core of a zombie stays in tact then changing the fringes is not uncommon.