I like to test things on YouTube so on Wednesday I came across this thread again and decided to give it a try on a video that I uploaded six months ago on my testing channel. The video was ranked on the 10th page position #6. Today (Friday) I remembered to check it again and the results are: page 9 position #14. That doesn't necessarily mean that changing a thumbnail will improve your ranking, but as I said before, whenever I've changed anything, whether it's something in the title, description, or even tags, there is a chance that your ranking will change for better or worse. This is why I recommend to leave a video alone if it's getting a respectable amount of traffic, because I've made the mistake of trying to improve an already successful video only to have it dropped from suggested videos, etc.