Thanks for the directions @JorgeB
I got things working, but not sure if I did it the best/right way. (Although if it's working, it can't be wrong, right?)
I tried flashing a new install to the usb key from the r710 that was tied to my Unraid license, but no matter what combination of uefi support I chose in the creator tool, or uefi support I enabled/disabled in the bios of the r720xd, I could not get it to boot from that usb key.
I was getting frustrated, so on a whim, I took the usb key I'd been using for the trial on the r720xd, and copied my key file over from the r710's flash backup. The r720xd booted off of that flash drive, it gave me warnings about the license, I went the replace key route, and everything is just working.
Thanks again!