I believe that the tensor product does not change precisely when the morphism of rings $R\to S$ is an epimorphism. The reason is that the tensor product over $S$ is constructed by balancing the tensor product over $R$ with respect to the $S$-action. But if the morphism is an epimorphism, the $S$-action is uniquely determined by the $R$-action and the tensor product over $R$ must therefore already be $S$-balanced.
I believe that the tensor product does not change precisely when the morphism of rings $R\to S$ is an epimorphism. The reason is that the tensor product over $S$ is constructed by balancing the tensor product over $R$ with respect to the $S$-action. But if the morphism is an epimorphism, the $S$-action is uniquely determined by the $R$-action and the tensor product over $R$ must therefore already be $S$-balanced.