Hi Martin,
> Before entering Downtime when SUM ask to Central Instance to be isolated and have a full Backup, Dataguard should be switched off?
You don't necessarily need to switch off the redo log transfer. It depends on your (network) bandwidth and requirements of course. You can also just stop the redo apply on standby, so the redo stream is still transferred but not applied until you re-enable it once again.
Oracle Data Guard is completely isolated from SUM upgrade activities.
Regards
Stefan