If we were to institute this system it is important that it does not lead to a wide array of too-short, or not truly constructive answers. There is a difference between an sufficient answer and a good answer, sometimes.
However, there is not always a difference between an answer short enough to be a comment and originally posted as such, and a good answer.
In the true spirit of democracy, preventing comments from ever becoming answers necessarily limits, in some cases, perfectly adequate and acceptable and constructive answers from being noted as such.
To allow the benefit while mitigating any potential harm, it is then necessary to put some checks or limits upon any such process of comment-promotion.
We already require self-answers to exist for 24 hours prior to their being capable of being accepted as the correct answer. And we already have structures that allow multiple votes to constitute a change, such as closing questions or opening closed questions.
If we were to allow comment promotion under the same principles it may work: Require 24 hours to pass between promotion and admissable acceptance as the answer, and allow downvoting to cancel out a promotion, say 5 downvotes or demote votes would send the answer back to a comment.