I went ahead and changed the NIC to the vmxnet3 adapter but it caused the network and sharing center to lock up the system. Instead, I installed vmxnet 2 (enhanced) and it went right into the network properties page. I re-assigned the IP information and applied the changes. So now, as usual, the system is working fine (for now).
Still would welcome feedback from fellow SBS2011/ESXi 5.1 users