As on the DC Comics page, which still reads "Cover by Neal Adams":
(via the DC Archives Message Board)
The title is different, too. It was solicited as "1050 Years in the Future" but it came out as "1050 Years of the Future" which actually makes more sense. Too bad nobody updated the DC page for either the cover artist or the title.
The Neal Adams original came from the 1976 DC Calendar (also seen, cropped, as the cover to a 1977 reprint paperback from Tempo Books), while the Grell original came from the Limited Collector's Edition #C-49, which reprinted the 2-parter of the Legion vs Mordru (and was also seen, redrawn, on the cover to Archives vol. 12).
In a kind of reverse Déjà Vu, this kind of cover swap was also seen last fall.