I've seen that it is possible to convert nook books into the mobi format that is readable on the
amazon kindle, via a website. I've also read about the drm beening striped in this porocess and thus being illegal. Ironically every question and answear I have read refers to converting amazon books to the nook, and being illegal. I assume this holds true vice versa, converting nook books to kindle, but would like to know for sure.
You can convert any open format eBook to another format legally. However, many eBooks are encumbered by DRM, thus making it illegal to convert those. There are tools out there that will strip the DRM from the file, all you have to do is look. If doing it for yourself the chances of being prosecuted are likely to be very small. If doing it to share with people the odds of being caught rise.