What additional functionality is provided by the F-14 nose pitot, absent in early models?
The probe on the nose isn't a pitot tube (although it might looks like one).

That's an AoA sensor as explained here: Home of M.A.T.S. - Sensor Probes

Reported from the link above:


  Angle-of-Attack Nose Probe: The Angle-of-Attack (AOA) is an essential dimension since it measures the angle between the aircraft's vertical flight direction and the vertical wind direction. A too high AOA means that at a given speed the wings will fail to produce lift which will result in a loss of controllability and stability. This is especially important during the landing phase where the aircraft is fairly slow approaching the carrier and also during air combat maneuvering where a loss of controllability can be deadly. 
   Each AOA probe measures the pressure at several points and then the AOA is calculated. Usually, several AOA probes are installed to measure a correct AOA.
   Note that this probe was not installed on very early F-14As


The same site, on a related page, report what you've alredy found:


  The first fleet Tomcats were not equipped with any nose probe, but todays F-14s carry a small nose probe to gather air pressure data to calculate the angle-of-attack