Quote:
Originally Posted by Moonraker
But then I couldn't work out how to get my new .opf file back into the zip file so I could then change the extension back to .epub.
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You unzip, edit the opf, and then rezip
everything (new opf included) into a new archive.
Or you use an archiving utility that allows you to edit files without unzipping them first (actually many just unzip stuff to a temp location and rezip things behind the scenes when you're done editing them, but the effect is the same--except the mimetype file will be compressed and possibly out of order, but Sigil will fix that when opening/saving).
A calibre conversion is another option for sure. But many want to keep the original code as unaltered as possible when downgrading from 3 to 2.
I myself, don't bother downgrading. I've yet to run into a commercially sold epub3 that didn't render acceptably in most popular epub2 reading systems. And if it's a "feature-rich" epub3, downgrading it will break its functionality anyway.