It downgrades in trial version because you did not remove all registry leftovers. When you remove it you will have no problems.
Already explained in the comments here and other EndNote's thread. Use REVO or IObit,... and Registry Finder to remove them all. It works fine you can even see a reaction of other people not so long ago.
I used IObit uninstaller 3 times to uninstall and then follow the steps. When you open the application it presents the error. I am sure I have not violated any instructions as I have doing so since 2008. I may give it another try. But I am sure that won't have any outcomes for me.