Old MacPro: Sierra on SSD, HFS+ RAID SET on four internal HDDs. How to install High Sierra?

Dear All:

Current configuration:

• MacPro 5,1
• SSD HFS+ (case-sensitive, don't ask 😠 ) with Sierra in DVD Bay ("System" drive)
• 4 HDDs in a HFS+ RAID set (also case-sensitive, in case it matters) ("Data" drive)

Need to upgrade to High Sierra installing on the same SSD drive, without having to reinstall all apps and recreate the RAID set.

Since my System drive is an SSD I understand I MUST reformat to the new AFPS first or else I can't install High Sierra. So, the plan is:

1) Backup my System disk with apps, etc to an external bootable Flash or HDD (using CCC so it boots?)

2) Boot into the NEW Sierra copy

3) Start the upgrade to High Sierra and choose the OLD internal System SSD as target, at which point I assume the installer will let me reformat it first.

4) Install High Sierra onto the OLD System SSD, now AFPS-formatted - a clean install with no old apps, etc.

5) Boot into the System SSD AFPS with High Sierra

Q1: Now, how do I copy all of my System files back from the Sierra drive?

If I CCC restore back to the NEW AFPS SSD, won't I end up with SIerra, not High Sierra on it?

Q2: Once Q1 is somehow solved, will High Sierra be able to ACCESS my HFS+ RAID SET without me needing to backup everything, break the raid set, re-format each of the four drives to AFPS, re-create the RAID SET and copy data files back.

Thank you, this is very confusing!

PS CCC being Carbon Copy Cloner, which I have and use already, any other migration suggestions are very welcome.