Why do you need to transition from jpeg to anything else? Just keep using jpeg for old files.

Chromium is literally the only reason jpeg-xl isn’t being adopted right now

That's not a "reason" it's a "decision". Their actual reason is pretty good — they don't want to support every image format that comes along. That's a slippery slope, there are several hundred image formats - should they all be supported? How many of them have security flaws? How much work is it to check for security flaws even if none exist?

The original image formats for the web, jpeg, gif, png, svg, all have major benefits compared to each other. That's why they were successful. There used to be other widely used image formats but they all fell by the wayside because the goal is to try not to have many formats. Ideally we'd only have one.

And WebP moves a long way in that direction, it does basically everything except vector images. AVIF is still around for efficiency reasons (it's very really easy/fast/low battery consumption for camera hardware to create an AVIF).

JPEG-XL has advantages but unlike those two they are really small and not worth the effort.