I recently upgraded from a t3i to a Canon 6D - 9& the t3i [???] I was able to digital zoom up to 10x and capture video using a 100-400 lens. I can zoom in using 10x digital zoom with the 6D, however, when I try to record at that zoom, the camera zooms back out (trying to capture the moon specifically) - I have searched high and low to determine what I am doing wrong but haven't had any luck with it. Anyone out there that can help me out?
I don't fully understand what you wrote. "9& the t3i"?
The EOS 6D lacks "digital zoom." As far as I know, the Rebel T3i also lacks "digital zoom." Both cameras allow 10x zoom on the rear LCD screen, for criticial focusing purposes, but both cameras only record at 1x.
Canon DSLR cameras don't even have optical zoom. DSLRs use detachable, interchangeable lenses, which are strictly optical zoom, assuming that the lens is not a prime. You must turn the zoom ring to adjust the focal length, not adjust the camera.
The Canon T3i ( or 600D in some countries) is the only DSLR that had this strange feature in video mode, it is something that was usually available in point & shoot cameras although the way it was implemented in the T3i was a slightly improved version.
It is generally not available in DSLRs because it causes a huge loss in quality of the final image and when people are serious about their photography and pay a lot of money for their cameras they certainly don't want gimmicky features that are going to reduce the quality of the final image to something they could have produced with a point & shoot camera or a mobile phone.
What you believe to be a digital zoom function on the 6D is just a system that allows you to zoom in the image for viewing or adjustment purposes it is not available for shooting and as you have found out it makes no difference to the final image.