To change the NIC in a VM log in as root and:
virsh list - This will list the running VMs
virsh edit (Name from the list above)
Now type '/ rtl <enter>' - That will get you to the rtl8139 entry
Hit "I" then delete rtl8139 and put e1000 in it's place
Now hit ESC and type ":wq<enter>"
virsh shutdown (name from above)
Once it is shutdown:
virsh start (Name from above)
If you do not see it in the existing VNC console close that window and click Terminal on the running VM in the appliance GUI. If you would like to do this once with us let us know and we will get a time scheduled.