I found another thread on this,
https://www.mobileread.com/forums/showthread.php?t=73314
That thread is Oct 2010, but it seems it couldn't be done back then either
The Ducks recommended using Sigil, I think I'll just remove the cover before I convert and put it back after, I had a look at Sigil a while back, can't see that
I need it - apart from this maybe, which I can do by delete cover and regenerate or with Tweak Book.