To what extent is it worth updating old thumbnails?
Lets say you have 400 videos in total, and about the last 50 of them contain your much improved high quality thumbnails. Would you go back and look at updating some of the older ~350 thumbnails to match? Or would there not really be much point?
Every video that you post comes with a thumbnail that represents you and that channel. Even if that video does not get views, your thumbnails sends a message to any visitors of your channel. In that regard, the first thumbnails people will see. On my desktop computer, the most recent 12 video thumbnails are displayed. If a viewer decides to scroll further, they will see 30 more video thumbnails depending on how far they are willing to scroll.
That means if you want to make a good impression, those most recent 30 or so thumbnails should be representative of whatever mood, feeling, message, you are trying to send.
So, I believe that updating thumbnails periodically is a good thing. I will prioritize the most recent ones then move to the most viewed ones since they get the most attention.
Regardless of the number of views, if a video is made public, there is a thumbnail that is conveying a message to channel visitors. Of course, the more videos you have the less impact any single thumbnail improvement will be. So people with fewer videos, each thumbnail carries more weight than someone like yourself with 400 videos.