Hi. I did something like this. I made an up to date image of Windows 7 before installing Windows 10. The upgrade had a few issues with installed software so I decided to then do a clean install of Windows 10 (Knowing I had my system image of Windows 7 to reinstall). Turned out there is no available graphics driver for the machine so I went to install my Windows 7 image.
I have to say my image was not made via Macrium or Windows, but via an inbuilt imaging tool (Intel) that came with the computer, which saves an image to usb. I couldn't boot from the usb stick so couldn't reinstall my image. I then wiped the disk but still couldn't get the usb to boot (have used this method many times before). I ended up reinstalling Windows 7 from the manufacturer recovery disk - and then couldn't get windows updates to work. It did work eventually after 10 days of tinkering about and trying fixes. And is now activated back on Windows 7.
So - I think there may be an issue reinstalling from a system image - but if you have a windows image and windows recovery disks it is more likely to work than my method. Activation may be an issue if the Windows 10 activation is still logged but phone activation could sort that out, explaining that Windows 10 has now been removed from that machine.
If you do the upgrade you have a month to go back to Windows 7 anyway - but of course if it doesn't go back well you may need your system image.
I don't think I would try upgrading unless I had either a recovery partition or recovery disks, or a copy of Windows 7 to install cleanly from scratch (back up your personal documents to reinstall later), just in case.
