Does changing a thumbnail after upload help?

Nothing resets. The video keeps its views, its watch time, its comments, its position in search and its place in Suggested. YouTube begins showing the new image on the next impressions and the old one disappears from everywhere except third-party caches. The fear that a swap will kill an already-working video is one of the most expensive myths in the space, because it stops people fixing the exact thing that is limiting them.

Whether it helps depends on one condition: is the video still getting impressions? A phone repair channel in G-9 Karachi Company had a battery replacement video sitting at 12,000 impressions a week with a 2.1 percent CTR — plenty of showings, almost no clicks. Swapping a dark workshop photo for a close-up of the swollen battery with the price in PKR was worth doing, and the CTR moved within a fortnight. A video getting 40 impressions a week has no audience to convert; a new thumbnail there changes nothing, and the honest advice is to make a new video instead.

Change one variable per swap. If you replace the thumbnail and rewrite the title on the same afternoon, you will never know which one moved the number, and on YouTube you will be making that same guess for years. Note the date of the change, leave it a full week, and compare CTR over comparable periods rather than day to day — a Sunday and a Wednesday are not the same audience. If your channel is eligible for the Test and compare tool in Studio, use it: it serves alternate thumbnails to real viewers and reports which one held watch time better, which is more reliable than your own opinion of your design.

We do a thumbnail pass on every new client's back catalogue in the first month, on the ten videos with the most impressions and the weakest CTR. It is usually the fastest win available, because the audience is already being offered the video and simply declining.