In that instance the actual answer text was changed to mean something else because of that edit (although the main text is still there the additional edit changes the meaning of the answer). Because people had already upvoted that answer in its original format then by changing it that means those voters have (by proxy) upvoted something they didn't mean to.
There is no problem with the OP leaving an answer to their own question. That's what really should have happened here.
For edits it doesn't matter who edits it; it could be the OP, any user, an anonymous user or even the person who originally answered it - If the edit substantially changes the meaning of the answer then it should probably be rejected.
However if the edit improves the answer (better formatting, adding examples or links to relevant materials etc) then it's fine to accept the edits. In fact such edits should be encouraged really.