I'm not sure if that will be an issue in practice. in any case, you need a JPEG XL decoder to perform the transition from a recompressed-JPEG-JXL to the original JPEG, so whatever tool is doing this, it can already handle native-JXL too. it could be the conversion happens on the server side and the client always sees JPEG, in which case a native JXL can also be decoded to a JPEG (or if lossless a PNG), though obviously with information loss since JPEG is a subset of JXL (to put it lightly)