Good post by Rawlinson Rivera (thank you)
Virtual SAN 6.0 introduced new changes to the structural components of its architecture. One of those changes is a new on-disk format which delivers better performance and capability enhancements. One of those new capabilities allows vSphere Admins to perform in-place rolling upgrades from Virtual SAN 5.5 to Virtual SAN 6.0 without introducing any application downtime.
Upgrading an existing Virtual SAN 5.5 cluster to Virtual SAN 6.0 is performed in multiple phases and it requires the re-formating of the of all of the magnetic disks that are being used in a Virtual SAN cluster. The upgrade is defined as a one-time procedure that is performed from RVC command line utility with a single command.
Read on here