So if everyone has to change to Mode S, can the current TCAS systems which presumably work with Modes A & C also work with Mode S or do they all have change to TCAS II or whatever it is and will TCAS II still work with any residual older modes?
All mandatory-carriage ACAS is TCAS II, and thus in effect all modern installed TCAS systems are already TCAS II. It uses Mode S or Mode C (not Mode A).