I haven’t found any specific information about this but I may have just been looking in the wrong places:
I owned End Note when I was completing a major project from 2003-2005; however I have since switched from a mac to a PC, and did not re-purchase EndNote since my institution provided a copy for me on my work computer. I have since moved institutions and would like to transfer my EndNote files from my 2003-2005 mac version to my pc. If I purchase the new version of End Note, will I be able to do this? Or are my old EndNote libraries lost to posterity?
You don’t say which old version, but if they are pre Endnote 9, later versions of endnote will convert them to the current Unicode compliant and database structure, creating a new library with the name “converted” appended (I think that is the word appended). The new library structure is not backward compatible however. If the file does not contain the .enl extension, appending this to the name will make the PC know which program to use. Remember also that later versions of Endnote have an associated .DATA folder which you shouldn’t lose. Keep it in the same folder location as the library (.enl) file itself. The current version of Endnote allows you to pack this together (or subcomponents of the library) as a compressed .enlx file from the File menu.