You can't directly change a NIC from one type to another. Not the GUI or API will allow this. The forbidden message likely comes along with the 'Cannot change network adapter type of existing virtual machine.' message as well.
The only way I can think of doing it is this and definitely test this on a non-critical or test system first.
1. Stop the VM in vCloud Director.
2. Capture the NIC config of the VM ... note the MAC address.
3. Remove the existing NIC in vCloud Director
4. Add a new NIC of VMXNET3 in vCenter and set the MAC as static and the original as captured from step 2.
5. Validate that the NIC appears in vCloud Director and that the network is correctly assigned to it.
6. Power on the VM in vCloud Director.
To be honest, I don't think this would work. vCloud Director absolutely doesn't like the VM configuration being modified in vCenter at all. vCD also doesn't allow Static MAC addresses on NICs to be determined by the User. This just isn't very "cloud like" ... the MAC should not matter in the grand scheme of things, but it really sucks when software binds to some specific hardware bit for licensing.