I suppose you could goto Calibre library, select all the books, under the Remove books drop down select Remove files of a specific format from selected books, and select mobi to remove the mobi files that are specific to your old Kindle. Download new copies with DRM specific to your new Kindle and then readd the mobi files to the appropriate books in the Calibre Library.
Of course, short of stripping the DRM, you will run into the exact problem the next time you have a new Kindle.
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