"I know I'll need some sort of software to convert the file..."
Ah, and therein lies the rub, so to speak. It is of course, illegal to defeat the DRM (Digital Rights Management) of an e-book. That's what converting a file (and the software that accomplishes it) often does. Yes, illegal even if you own the e-book in another format.
Far be it from me (or probably others on this forum) to recommend or advise anything to assist you in performing such a heinous and naughty act. However, a well-formed Google search yields all sorts of information on so many fascinating subjects ...
