I didn’t know this was a thing I wanted to do, but while upgrading a few Cisco 3650’s I accidentally threw on the wrong bin file. I pointed to the wrong bin file and rebooted. Luckily I had a console cable plugged in and ready already. I knew the previous image was good, so now that I am stuck at rommon, or a switch: prompt I was able to enter this and it booted right into the old and good ios:
I don’t know why this was so hard to find, but googling for ios upgrade revert rommon, hopefully this comes up and saves you some looking!