Thank you so much, Mongorhead, for doing this modification! It's working just fine on my Windows 7 32-bit system with calibre 4.23.
(And thank you, JSWolf, for putting us on the right path.)
Mongorhead, you're right that a .kepub file is produced and saved on your hard drive. I just clear them out periodically by entering this in the "Virtual Library" line at the top of the Ccalibre screen:
Then used Ctrl-A to "Select All," and then right-click and choose "Remove books," and then "Remove files of a specific format from selected books," and then choose kepub.
All best wishes — and thanks again!