I've succeeded in migrating virtual machines from standard to distributed switches before, without downing them. It involved making sure there were at least two physical connections on the host to the physical switch network, then having one connection on the standard switch and one on the distributed switch. You lose redundancy for the short term, until you complete the migration and then switch the remaining physical connection over. VLANs also worked.
It was trickier to do the vKernel ports, but not impossible. There's a migration wizard in the vCenter client for that, and the wizard does standard-to-DS or DS-to-standard. The same migration wizard can do VMs so one could swap several VMs at once.
When I first set up DS, I did take the hosts into maintenance mode one at a time, but I had to do the split networking anyway or vMotion wouldn't work moving VMs to a host with new DS switches. So I ended up having to do the work outside of maintenance mode with live VMs running on the affected host. You can bet I was damn sure connectivity was maintained during that migration. In any case I'd probably vMotion the VMs out, take the host into maintenance mode, set up the split environment, migrate the vKernel ports, then take the host out of maintenance mode, vMotion the VMs in, and migrate the VMs.
And looking at this, I'm thinking that in our environment distributed switches just aren't worth the hassle or the license for Enterprise Plus. Maybe if it's needed for fault tolerance or high availability or something, I can restore DS later, after I finish the migration to vCenter 5.5.
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