If your proposed answer adds constructively to (and differs enough from) the existing content, either:

- go ahead and post an answer, irrespective of whether the existing answer is upvoted, accepted, etc., or
- ask the OP to add the things you'd like to have in the answer, and if they can't, or didn't after a reasonable time, add your answer.

What I'd do in such a case is comment first. I know editing to improve answers is the way of life in SE, but making large scale changes to another's answer makes me uncomfortable, especially since my edits won't go through review. If the OP then gives me the go-ahead, then I make the edit.