Maybe not the answer to your question, but did you consider to use another SD card for the new version?
Depending on the complexity of your setup, this might be the best option anyway, and lets ESXi 6.7 create its additional partitions (compared to ESXi 5.x).
This way you don't need to backup/restore the curent SD, and you can always just insert it again, if something doesn't work as expected with the new setup.
André