I tried an update using the usual Unibeast method, but it would not boot after update.
Where do you use the iMac 13,2 definition?
I did the upgraded differently since my SSD was dying.
1) Made a backup of the MacOS drive. (Using CarbonCloneBackup, you can find the free version before they started charging for it) Create a sparse bundle image.
2) Create USB High Sierra disk using UniBeast and complete the install on the new drive.
3) iMac 13,2 definition is from the SMBIOS. You can generate a new one using MultiBeast or Clover Configurator.
4) Migrate data using the Migration Assistant.
For your case. I would do the following:
1) Change your mac definition using MultiBeast or Clover Configurator. For our system, I think the closest is iMac 13,2. In MultiBeast, select:
Customization > System Definitions > iMac > iMac13,2
Build > Install
Reboot
2) Switch from Chimera to Clover. Make sure you can boot your current system using Clover.
3) Make sure you only have one drive installed on your system. (Disable everything in the BIOS except for the MacOS disk). Use GPT instead of MBR.
4) Use the UniBeast USB disk to upgrade.
I did those steps before when I was testing and it worked from Yosemite --> El Capitan ---> High Sierra. I didn't have an issue. Just remember to only have one disk in your system with MacOS when doing the install or upgrade.
One thing I learned is to always have multiple installs of macOS running in separate physical disks. This way you can test the upgrades/updates before changing your main system.
