I converted a number of Nook books I had purchased to kindle, and it was easy, and I imagine it is pretty easy the other way too. Because of the rules here, which as noted above by a moderator, are unambiguous on what can and can not be discussed here re: DRM removal, I simply did googled Apprentice Alf, found the instructions and went from there. I will say the first time I tried it I had some problems, probably similar to what Katia is talking about.
Then I googled how to strip DRM from books, looking for mac apps, found a wonderfully simple program, and then was able to convert my books easily through that program and Calibre.
I'm only moderately tech savvy, and I figured it out, so it can be done. As they say, Google is your friend!