Each vSwitch - Standard and/or Distributed - needs its own uplinks (vmnics), so unless the hosts have unused network ports, you need to do a migration. As a side note, if the physical switches are the same for Management, vMotion, and VM traffic, you may consider to merge the different networks into one distributed switch.
Please don't mind me asking, but what's the reason to move from virtual switch tagging to virtual machine tagging?
André