If they are ePubs, see
Adobe ADEPT DRM for EPUB circumvented. Note that in the US, for example, this is likely legal for personal use of ebooks you bought. Once DRM-free, Calibre can convert from ePub to MOBI for the Kindle. This conversion is generally very good, as good or better than the version you get from the Kindle store.
If you bought almost any other ebook Reader, the original DRMed ePubs would work without modification. However, I would still strip the DRM - because it will inevitably screw you eventually. Since the DRM free ePub to MOBI conversion works so well, you can buy a reading device without worrying about compatibility with your existing ebooks.