Migrate all VMs off ESXi host.
Put ESXi host into maintenance mode.
Go onto ESXi console and reset network using the "vSwitch" option.
Configure the new Management switch with the required ports, attach the extra uplink that should be using it.
Remove one uplink from the vDS and create a new VM network vSwitch using the removed uplink.
Migrate some VMs back onto the ESXi host.
Select each VM and change the port group to one that is one the vSwitch.
Once all VMs on the host are using the vSwitch, remove the last uplink from the vDS and attach it to the vSwitch.
Repeat this process for each host.