Thank you so much for gathering all of this information in one place. I personally had been using the VMDK boot method but updating was a huge pain. I figured I would share my journey to move from VMDK boot to Plop boot.
Fist of all thank you to Itamar for his instructions on setting up Plop.
Follow all of the steps under "Make a Plop Boot Manager image (on Windows)"
Copy the ISO to the VM folder on the datastore.
Copy all of the files from the boot VMDK to USB
I had an issue with syslinux, I kept what was on the USB
Remove Hard Disk 1 (old boot VMDK) from the VM, do not delete from disk yet.
Add 32 MB boot image to VM (thin)
Increase memory to 2+ GB, this is needed to update unRAID via the GUI.
Configure the VM to Boot from ISO located in the same folder.
You can see detailed steps on Itamar's site.
In unRAID validate your Disks and Configs
Delete the old boot VMDK from disk
Once my system was here, I couldn't stop myself so I updated via the GUI.
Make USB bootable via BAT file (run as an administrator)
I am not sure if this is required but my system would not boot after the update until I made it bootable again
I hope this helps anyone else changing boot methods.