Hi,
You can try and migrate the physical NICs to the distributed switch on portions. First connect one physical NIC on each host to the distributed switch. After you make sure that every host has at least one physical NIC connected to the distributed switch, you can migrate the rest of the NICs to the distributed switch. Does that make sense?
Here's another link that might have useful guidelines as well:
http://www.vmware.com/files/pdf/vsphere-vnetwork-ds-migration-configuration-wp.pdf
Regards,
Dobrinka