That is because Endnote keeps the citation information in a “traveling library” associated with the paper, and if you change the year, it thinks it is a different citation and uses the info it saved in the traveling library rather than the one with the mismatched year in the library. Does Basic allow you to “convert to temporary citations”? If so, when you “update citations and bibliography” Endnote should prompt you to replace those citations that don’t match anything in the library and you can select the altered one.