I'm going to go against the grain...I updated a number of my thumbnails and those videos have been suddenly doing a lot better than when I originally posted them a few months ago, so it can be worthwhile to update them. Sure, you can see the progression of who you were at the beginning, but that's only if someone comes directly to your channel, but what if your thumbnail comes up in search results? You want to put the best face forward each time someone reaches your content, no matter how they find it.