In fact, I'm pretty sure that explicit labeling (such as exhibit A) devalue the brand. Especially on a Guinness container. Who cares that the look has changed? Shouldn't we notice the change and exclaim with tears of joy streaming down our face, "Look! They have a new look!"
It's almost akin to prefacing a joke by saying "this is really funny, you're going to love it."
In sum, new looks should not include stamps that say 'new look.'