The JPEG -> JXL conversion is reversible, so you can encode JPEG -> convert JXL -> convert back to JPEG as needed. You could potentially encode a JPEG-XL directly with the subset that can be converted back to JPEG, but it's not clear to me if libjxl currently implements that.

Either way, it's not that useful for anyone that's already deploying post-JPEG formats; even WebP should save more than 20%. Mostly it's useful for CDNs and SaaS companies like Cloudinary to transparently deploy JPEG-XL for JPEG-only customers.