I have the latest version, and it works perfectly: original_epub is preferred to epub, if it exists.

I do have a question regarding other formats: basically, I'm extending my original question to conversion from any format to epub, always using the original for later conversions. After poking through the source, I've come to the following conclusion:

If you perform an individual conversion of a book, it stores the source format in the gui_preferred_input_format field in the conversion_options table.

If you perform a bulk convert, all books that have gui_preferred_input_format set will use that format (so if I convert a .mobi to .epub, bulk convert will always prefer .mobi for future conversions). Books without gui_preferred_input_format will use the priority list.

This means books will always be converted from the original format, unless I delete the associated rows in the conversion_options table. This is important for me because I'm planning on deleting all rows from that table to clear out custom conversion options I may have set while experimenting.

Is my understanding correct?