Just to add that for me renaming the AZW3 file to AZW allows me to add it to Calibre, BUT the AZW file cannot be read on my Kindle Touch. But if I move the AZW3 file to my kobo touch with the AZW3 extension, the Touch can read it.
So we need a way to have Calibre handle the AZW3 files with the AZW3 extension. I can add AZW3 to calibre, but then it refuses to send it to my Kindle touch, saying the format is not compatible
So for the moment the only solution to me is to keep the version with both KF7 and KF8, but the size is then twice the size of an ePub