Just a quick FYI for any that might be curious. After good results with the previous (and current) public beta on my 2016 MBP, I opted to move my late-2014 5k iMac to the current public beta (17A344b). My system has a 1TB SSD internally and has a direct attached, Thunderbolt 2 RAID array (Promise2 R6: 6 3TB drives in RAID 5 configuration yielding 15TB of usable disk) that has standard HDDs inside. I left the RAID array attached while performing the upgrade. Everything went smooth and when the process was completed, my internal SSD was converted to APFS (Trim enabled) and the attached RAID was left as Mac OS Extended (Journaled) as I would expect. After 24 hours, everything running great.