I was able to update from Yosemite 10.10 to Sierra 10.12 without any problems. And then, applied the update to 10.12.6 from the App Store.
The update to High Sierra was a bit different.
- Create installer USB drive with Unibeast
- Power off machine and disconnect HDD with home folders
- Boot from the Installer USB drive: make sure to use the machine's boot selection option to do so
- At the Clover screen, go to Options and set the following
- Boot options: dart=0
- SMBIOS: model is iMac13,2
Run the macOS Installer/Updater, targeting the system drive. After a few minutes, the screen will go black in a reboot attempt. Manually press the reset button. Use the firmware boot selection to pick the Installer USB drive as boot drive. At the Clover screen, set the same options/settings as above. Then, select the "Install macOS" boot drive that is on the system HDD (not the USB drive) to boot from.
The installer/updater should come up and continue the upgrade process. It will be a white or light grey screen, with a grey Apple logo, and a progress bar saying "Installing: About X minutes remaining".