The old version of the function could get security patches. But then it wouldn't be the old version of the function any more, but a brand new function. If you

The old version of the function could get security patches. But then it wouldn't be the old version of the function any more, but a brand new function. If you allow arbitrarily changing 'old' function then you just lost all the guarantees of stability and reproducability that this approach was supposed to offer.