Look at coke. It tastes the same; today and yesterday. It had a couple of sub character products; e.g. Vanilla coke.
2nd example is Nandos. Changes marketing campaigns more than we change our underwear. But their chicken is the same throughout. Oh their dressings change.
They rule is there is no rule but a rule. The safe thing to do is; rather introduce sub character products. Sub character products = sub products.
Our guess is; improvement is only allowed to mean change in taste; if maybe laws or health advances necessitate this. But it has to be carefully weighted. Is this improvement really really a must; ask yourself that.