DO NOT UPGRADE!
Endnote 20 gutted all of the best features of Endnote: you can’t preview PDFs in the same window as the references; you cannot have multiple references open at the same time (in tabs or windows); and the interface looks amateurish at best.
Stick with Endnote X9. Unless there are huge future changes which correct the direction they have gone, it will be the last version of Endnote that I ever use. It really is sad.