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calibre can convert from one format back to the same format. That is exactly what people who want to replace MOBI covers typically do.
You can do that via the cli.
Or in the library GUI -- in which case you will end up with an ORIGINAL_FMT and a FMT.
The ORIGINAL_FMT is a backup and is used for future conversions to reduce the lossy nature of converting again and again.
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