[DiapDealer]

The short answer is that you can't prevent calibre from renaming classes (though it will never name one MsoNormall) if you do a conversion. It's conversion, after all, not editing. Something entirely new will always be the result of a conversion.

As you've seen however, those changes don't really result (usually) in any difference in how the document renders. You can go back and change the style names if you like, but as theducks mentioned, you'd have to change all the corresponding class attributes in the html as well.

If maintaining existing code (html, css, and class names) is a priority of yours, then you need to build/edit the epubs yourself with an editing program (calibre-edit, Sigil) where you can maintain that level of control--rather than converting.

EDIT: also make sure you're not being fooled by the ORIGINAL_EPUB automatic backup thingie. It wouldn't be the first time it confused someone as to what the real source was for the conversion they thought they were performing.