Network Interface Card (NIC) properties cannot be changed with outdated XenServer Tools.
Cause
Once the XenServer and XenServer Tools are updated you are unable to edit the properties of the NIC when the XenServer Tools on the Virtual Machine (VM) are out of date.
Resolution
After applying an update to your XenServer (and updating the XenServer Tools in the process) you receive the notification in your XenCenter console that the XenServer Tools are out of date for the Virtual Machine(s) (VM) hosted by that XenServer.
When editing the NIC settings of a VM in the XenCenter console you are unable to select the Properties, Remove and Deactivate buttons. You are only able to add a new NIC to the VM.
After upgrading the XenServer Tools in the VM and rebooting it, the message in the XenCenter console no longer appears. Also, now you can edit, remove, or deactivate your NICs.
More Information
This issue was seen on a XenServer 6.0.2 after applying updates CTX132823 – Hotfix XS602E001 – For XenServer 6.0.2 and CTX132824 – Hotfix XS602E002 – For XenServer 6.0.2.
You can read the original post (written by myself 
) here on the Citrix knowledgebase.