If you can go both directions, you can store the more efficient jpegXL format but still have perfectly transparent support for clients that don't support jpegXL.
If you can't produce the exact same original jpeg, then you can still have some issues during the global upgrade process -- eg your webserver database of image hashes for deduplication has to be reconstructed.
A relatively minor problem to be sure, but afaict if jpegXL does support this (which apparently it does), the upgrade process is really as pain-free as I could imagine. I can't really think of anything more you could ask for out of a new format. Better & backwards+forward compatibility
