I also on a side note believe that you still need an underlying qualifying OS, either Win 7, 8, 8.1, OR 10 Pro to upgrade to Enterprise, so they were likely in MS licensing violation if they didn't provide a retail or OEM Win 10 license with them.
Try the WMI or Powersehll command I provided above, if you get no response or empty then there is no OEM key in the BIOS, if this vendor did not provide you an OEM license which is possible, then you will need to as I said buy 15 retail Pro licenses of via VL get 15 get-genuine kits. How do you revert back to the OEM? As I said, I do not have any product keys. I'm still ignorant on how to change the license key back to OEM status for the computers in question.
Excellent advice Alex! I have the paperwork in front of me, and we are indeed getting charged $36 /month PER computer on a monthly basis.
