Microsoft has just updated the following KB Article:
The Apply button is unavailable after clearing the “Enable PXE support for clients” option on a System Center 2012 Configuration Manager Distribution Point
Symptoms:
When you clear the Enable PXE support for clients option on the PXE tab in the properties of a System Center 2012 Configuration Manager Distribution Point, the Apply button is unavailable.
Full details at:
https://support.microsoft.com/en-us/kb/2832598