user-1: My perception about the quality of an answer may change from positive to neither positive nor negative. The answer is subject to change as well. In either case, one might want to un-upvote the answer, however, it seems it only seems possible either upvote an answer or downvote an answer. Is un-upvoting a possible feature or privilege to unlock?

user254: You click on the up-arrow to upvote an answer: and click on the up-arrow again to cancel the upvote.
See Is there a way to cancel upvote on answer / question?

If I try to un-upvote an answer which I previously upvoted days ago, I see a popup message like,

You last voted on this answer at [date and time]. Your vote is now locked in unless this answer is edited.

In summary:

After I upvote a answer I have a finite time during which I can change my mind and un-upvote it
After that time (I'm not sure how long it is, maybe only 5 minutes) I am not able to un-upvote
If the answer is edited again after you upvote it, then un-upvoting becomes possible

If you edit the answer yourself then you'll be able to un-vote it (but doing that seems a bit hostile and should usually be avoided).

-1: <p>My perception about the quality of an answer may change from positive to neither positive nor negative. The answer is subject to change as well. In either case, one might want to un-upvote the answer, however, it seems it only seems possible either upvote an answer or downvote an answer. Is un-upvoting a possible feature or privilege to unlock?</p>
 254: <p>You click on the up-arrow to upvote an answer: and click on the up-arrow again to cancel the upvote.</p>

<p>See <a href="https://meta.stackexchange.com/q/22473/139866">Is there a way to cancel upvote on answer / question?</a></p>

<hr>

<p>If I try to un-upvote an answer which I previously upvoted days ago, I see a popup message like,</p>

<blockquote>
  <p>You last voted on this answer at [<em>date and time</em>]. Your vote is now locked in unless this answer is edited.</p>
</blockquote>

<p>In summary:</p>

<ul>
<li>After I upvote a answer I have a finite time during which I can change my mind and un-upvote it</li>
<li>After that time (I'm not sure how long it is, maybe only 5 minutes) I am not able to un-upvote</li>
<li>If the answer is edited again after you upvote it, then un-upvoting becomes possible</li>
</ul>

<p>If you edit the answer yourself then you'll be able to un-vote it (but doing that seems a bit hostile and should usually be avoided).</p>
