All upgrades can be done online and they are not service affecting as long as you have your failover settings setup correctly within your OS's, HBA cards, etc. and you are running dual controllers. :-) As most vendors say... they do recommend doing upgrades during a lower system utilized period and during a maintenance window (not outage, just maintenance) in case anything issues arise during the upgrade process.
We are planning to upgrade the SCOS within the next couple of week. Do Dell recommend that we take all HyperV servers down whilst we upgrade or can we leave them up and running?
SCOS Upgrade - Do we need to take all servers off line?