Ok, great, thanks; that appears to be the equivalent of dropping the row from the database.

If so, that would have the same issue of forgetting my preferred input format, right? So if I have my "master", say, .mobi that I'd converted to .epub, if I restore defaults, Calibre will now convert from .epub to .epub by default (since epub has a higher priority as an input than mobi).

Is there a way, independent of other conversion options, to force a particular book to have a "master" format? E.g. I'd like, when I add a book to Calibre, to set the book's original format as the master format, even if I hit "restore defaults" later on. If there's not a way to do this currently, I think it'll be easy enough for me to hack into Calibre, which raises the question of whether I should send a resulting patch in, or whether it's too narrow of a feature to really be useful for anybody else.