We are not lawyers, but I am in the business of selling software, and hence have a good "practical knowledge" of what copyright law permits one to do, and not to do, with digital media (eg I recently successfully went after someone who was re-selling my software on eBay).
As you say, though, we can certainly agree to differ. As a purely practical matter, nobody's going to come after you for converting the format of eBooks that you're legally purchased. Just don't try to re-sell them
.