Doesn't matter anymore. I ended up bulk converting the whole library, after having made a backup.
Could not make send to device work with the Sony Tablet S, so I saved the lot to a single directory (couldn't use the (USB)device as target) and used common file transfer to send the books to my tablet.
Now I just need to find out which books didn't convert - about 93 out of 1339. Anyway, I
should have plenty of reading for a while
BTW: Send to device may be hard to get to work with android devices, as the target folder depends on which ebook reader we use. The various reader applications I've tested - "Aldiko", "Cool Reader", "FBReader" and the built-in "Reader" - all use different folders to store their libraries. However, Aldiko readily imports from the Download folder - don't know about how the others do.