Originally Posted by itimpi
...it is easy to remove a particular format. This can be done via the Bulk Edit Metadata dialog. However to avoid removing the only format you currently have, you might want...
THANKs for the thoughts !
i found that FIND -> formats:"=EPUB" and formats:"=MOBI"
= gave me books which have BOTH epub AND MOBI, so, next step is easy - delete specific format from all marked books...
Calibre is GOOD