[itimpi]

There has been discussion of Calibre being enhanced to have the idea of an 'archive' or 'master' format of a book that would be stored like any other format is at the moment. This would contain a format of a book that would never be over-written during conversion, so it could fulfil the role you are looking for.

However, as far as I know, this has not yet moved beyond the concept stage. I think that this is because although the concept is simple it touches on lots of areas of the Calibre code so the implementation would be a non-trivial task.