I don't think there's a hard and fast policy either way.
Personally, I think that if you're going to totally (or almost entirely) overwrite your previous answer because it's invalidated by a change, you should simply write a new answer (and maybe delete the old one). The votes on the answer were on an answer that is totally different from its current form, for instance.
Alternately, if your old answer is invalidated by the change but you still want to acknowledge that there was a change in the rules, you could edit your original answer to state the new/current answer front and center, and then briefly summarize what changed/what the old answer was before that change.
Either way, whether you edit the original answer or simply post a new one, the question will be pushed back to the top of the front page when there's answer activity (whether it's a new answer or an edit to an old one).