Reconnecting your Kindle triggers calibre to update the cover thumbnail images for sideloaded books. That is done to work around a problem where the Kindle will sometimes remove those images when it connects to Amazon’s servers. They are stored in a thumbnail cache on the Kindle separate from the book files. That action by calibre has a coincidental side effect of updating the cover thumbnail if you have changed it.
To change anything else, such as the title, author, or cover actually contained inside of the book you have to delete to old book from your Kindle and send the new one.
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