Migrating ESX host and VMs from one Vcenter to another

I need to move few ESX hosts and its VMs from one Vcenter to another. I don't want to get a MAC address conflict for the new VMs creating in old Vcenter with VMs migrated to new Vcenter. What are the MAC prefix i can assign in old Vcenter other than default OUI 00:50:56.

My environment: Vcenter 5.1, ESXi 5.1 and Windows 2003 & 2008 VMs.