Of the school "if it ain't broke..." BUT there are numerous indicators I should upgrade to High Sierra from Sierra (mostly bug fixes in Metal). Yes I have a plan (strip everything from the machine except a single drive (which has a clean Sierra ion it), put in the OEM GPU, update firmware and the single drive, put my 980 back in, do the Mojave updater ONLY for the 144.0.0 firmware update, put everything back in).
Now, my boot drive is an array of SATA SSDs on a PCI card. This array I DO want to update to HS.
1. will it force me to go APFS (I suspect not because of it being an array)
2. will it refuse to update for any other reasons (which are)?
My fall back plan is I have a clone of my boot drive on a spinner. Theoretically, I can boot from that, update to HS, then clone it to the array. Anyone see any issues there?