With all of the Mode S stuff coming the testing tolerances are apparently tighter, which will force owners to upgrade their transponders
Mode-s is quite not the same as ADS-B. There are Mode S transponders that will be redundant as soon as ADS-B becomes mandatory.
Currently, if you want to fly IFR anywhere in Australia at any level in Feb 2017 you will require a Mode S ES ADS-B transponder with a C146a GPS.
We will be the only country in the world to require ADS-B for all IFR flight at all levels in all airspace.