I too am unsure of the upgrade path there.
I am using VSC 6.2.1 to perform backups (and update SnapMirror and SnapVault destinations) in my vSphere 6.5 environment.
I plan to upgrade from ONTAP 9.1P8 to 9.3, which requires VSC 7.1. This facilitiates moving to SnapCenter for the backups. The documentation is not clear on how you actually upgrade and how you retain visibility of the VSC 6.2.1 backups so you can continue to run backups and restore the older backups that VSC created.