Shut down the virtual machine you want to migrate.
In VMM 2012 > ‘VM’s and Services’ Right click on the virtual machine, click properties.

Click “Hardware Configuration”
Scroll down to Network Adapters and choose, you network adapter.
Under connectivity, choose “Not Connected”. Click ok.
You should then be able to migrate the virtual machine.
Once the migration is complete, go back into the Network Adapter properties.
Click “Connected to a VM network”
And choose a VM network available. I didn’t worry about the VM Subnet section and just left it blank. Click ok
Turn on the virtual machine. Complete.