Francois,
Just a suggestion. Before deleting the unwanted formats, make absolutely sure that your conversions are acceptable, word for word, paragraph for paragraph, chapter for chapter.
Sometimes the ebook source file is such a mess that calibre doesn't produce perfect results. I'm not trying to slight calibre, just recognizing that the GIGO (garbage in, garbage out) rule applies here. Unfortunately you might only notice less than perfect results after you have deleted the original ebook source. Leaving your only option to manually edit the ebook. That's a lot of work and it might introduce new errors.
If you have the HDD space, or lots of DVDs available, keep a copy of the entire library as it exists now - including the original, unwanted ebook formats. It never hurts to have a back-up.
