A video isn't set in stone once it's published. As long as it shows up in search or suggestions, its thumbnail keeps working. And you can change it anytime from YouTube Studio.
The videos worth redoing
- The ones that still get impressions: a video YouTube no longer shows won't benefit from a new thumbnail.
- Evergreen topics: tutorials, guides, reviews, stories. People still search for them months later.
- The ones whose thumbnail has aged: the channel's old style, a low-quality image, text that no longer fits.
What not to change
The new thumbnail must always match the video. Promising something other than what it shows makes people leave fast, and YouTube looks at what happens after the click. Keep the same topic, change how you show it.
Measure the result honestly
Compare periods of the same length before and after the change, and look at impressions together with click-through rate. If impressions moved a lot, the comparison becomes misleading (see why CTR moves with impressions).
The most reliable option is still YouTube's A/B test: the old and new thumbnails are shown in parallel, and YouTube compares the watch time of each version. It also works on videos that are already published, as long as they're eligible.
Keep a record
Write down every change: the date, the old thumbnail, the new one. In a few months, you'll know which changes worked on your channel, and why.