Moderator: Thread moved to the vCenter Server area.
I have several times enables or disabled vCenter HA in a production environment, there is no outage.
And from my experience i can say: Don't use the H5 client, use the flash client for the inital setup.
As Raudi said, there is no downtime...
But if you want to see it by yourself, the next is the official video of the feature in a walkthough of the configuration process (you will se that there is no outage)
VCSA HA doesn't allow for patching with no downtime, then you have to reconsider deploying it. Just enabling it and it doesn't cause an outage .