Moderators have the ability to convert answers to comments when necessary. It would be nice if upvotes on the answer were automatically transferred as upvotes on the comment: sometimes, a person will post an answer which raises a very important, relevant point, but isn't an answer. In those cases, it would be nice for the information to still be highlighted. If a comment thread is over 5 long, some comments aren't shown, so giving the comment the votes from the answer it came from would be useful.
For example: This answer on Super User (now a comment, for those with <10k) provided some useful advice: "be careful not to bend the pin too many times, they turn brittle quickly and snap". Should it have ever been an answer? Probably not, but the user was providing helpful information, and at the time had <50 rep, so he couldn't answer. The point here is that if an answer should be a comment and it has a lot of upvotes, it's probably important in some way.
Because comments do not have such a thing as a "downvote", this could perhaps only be applied to comments with no downvotes, or the total score (
up - down) could be used. To prevent users from re-voting on the comment, anybody who had already voted for the answer would see the UI as if they had already upvoted it. Prior downvoters would retain the ability to vote it back up.
Based on some of the comments and related answers suggested below, I would also propose that this only be applied when the comment has a total score of at least 5, indicating that it was actually useful in some way. Again, this is so that potentially important information doesn't get burried, even if it shouldn't have been an answer in the first place.