Whenever you create a new Virtual Machine for an existing (previously configured) VHD (either by copying the VHD or using differencing disks), the networking configuration of the VM is reset.

This is because when creating a new VM, the Network Adapter(s) you assign receive a new Guid and are treated by the OS as new hardware.

The following post explains this behavior in detail:

http://blogs.technet.com/jhoward/archive/2008/07/22/hyper-v-why-is-networking-reset-in-my-vm-when-i-copy-a-vhd.aspx