Not quite. He had already photographed some American independents in the late 60s, and in the 70s he had been hired for Monte Hellman's Cockfighter (1974) prior to Days of Heaven. I'm just now watching Hellman's Two-Lane Blacktop (1971) btw. Maybe I should check out Cockfighter next.

The problem I have with Almendros' films on Video or DVD is that imo his lighting of scenes (or his use of natural light) can be crucial to the experience of the film, as it isn't only an aesthetic choice but influences my perception of the character's feelings, sets the mood for the whole film. I recently watched the introspective Mes petites amoureuses (1974) again, and the difference between the mediocre image on DVD and the exceptional beauty of the 35mm print I had previously seen was striking. One totally couldn't see what was so exceptional about his work on this one, and I would say that his cinematography for Eustache is some of the best you'll ever get to see! 

After this experience I kept wondering how many films I had failed to appreciate because I hadn't been able to watch a good copy of them. It's like Van Gogh's paintings without the vibrant colors! 

So we have to be deeply thankful to companies like Criterion who try to offer us the best possible image and audio (another crucial aspect) they can get. Still more important though is the work of archivists and restorers who take care of the original prints. For to see a film as intended by its creators and in its full glory you have to see it on film. So I'm not sure if HD will actually be such an improvement, at least for the vast cultural legacy which films from the 19th and 20th century represent.