I call it archiving.
versioning is wanting two or more files of a given format, not one of each.
this is not versioning, it's diversity
1 x file.original_epub
1 x file.epub
1 x file.mobi
1 x file.azw
with all 4 selectable as source formats for conversion
versioning would be
file at date a.epub
file at date b.epub
....
the original_FMT files behaves like an archive format in most instances already e.g. it cannot be opened in calibre viewer, or in calibre editor....