A perosn I work for is interested in buying a copy of Endnote. She owns Endnote 6, and is interested in upgrading to X6. Is she still able to upgrade with a version that is that old?
Best to ask customer services here, but in my experience, they are usually accomodating.
She will need the original key which is some thing like 9 numbers in the front cover of the paper manual (which they still had back then or on the CD envelope, or the back of the packaging box) or if installed, the number is in the About screen.
Write it down before uninstalling. And it is highly recommended that you uninstall the old version before installing the upgrade.
You can upgrade from any prior version of the software to the current version. As Leanne mentioned, you will need your prior serial number to complete the upgrade installation. See the following article for details:
Technical Sup Rep RS
Phone: +1 800-336-4474