Evening
dvSwitche configuration is stored in vcenter. Each ESXi host holds portions of the switch which relate to it's virtual machines. This enables the switch to operate when vcenter is not present. So if you are doing a rolling upgrade without loss of vcenter you will be fine. If you follow this process you should not have any real downtime:
1. Upgrade to 5.xxx with current vcenter
2. Move a single with a spare nic into new vcenter
3. Add spare nic to new vcenter's dvswitch (assume that dvswitch has the same port groups as old)
4. Migrate vm's one at a time to new switch. This migrate will cause the vm to be unavailable for a few seconds.
Let me know if you have additional questions.
Thanks,
J