I direct your attention to this answer which has been sustained and updated over time. Reading through the answer shows how times change and improvements warrant readdressing questions and answers. In this case MATLAB has improved their engine over time and people who visited this question years ago leave with a different understanding than those today.
In cases like this of ongoing support and effort on the part of the user answering the question, I feel it's appropriate to award with multiple upvotes. Each update required some level of effort on their part and because it adds significantly to the answer, should be rewarded.
The alternatives I see is for the user to either make a new answer which will be relegated to be lower than their first answer, or delete the current answer and lose the previously gained reputation. Both of which seem to hurt Stack Overflow and the users.
It seems like a simple check on the amount of content changed/added could be done to allow an answer to be eligible for additional upvotes. I don’t see any reason to allow additional downvotes as an answer doesn't get "more wrong". I can already change from an upvote because of an edit.
I think in cases of ongoing support and in the interest of providing sustained quality, users who add substantial content to existing questions should be rewarded. What are the counterarguments or reasons for not doing this?