Fuji,
I shall look forward to hearing more about the changes. I hope that these arent rolled out with the usual lack of consultation that seems endemic in aviation.
There shouldn't be any consultation necessary as what is hopefully coming is something that is actually wanted by the majority of the GA community I'd say.
I was and still remain opposed to the compulsory introdcution of mode S. I do not see it offers any material advantages to GA over mode C. However my view on transponders has changed. In my opinion all aircraft should be fitted with a transponder. The fact of the matter maybe that mode S transponders will become as "cheap" as mode C before to long.
It's actually an ICAO requirement now, has been for some time, that all aircraft should carry an altitude reporting transponder so mandatory carriage has always been on the cards, just that no-one took much notice of it.
However if all aircraft were to be fitted with Mode A/C transponders the simple fact is the infrastructure couldn't cope. One of the reasons for mandating S instead of A/C is that the nature of S allows more transponders to be in the air at any one time than there ever could be with A/C without overloading the system.
In short mandate A/C, the sytem can't cope. Mandate S it can.
No doubt a mix of A/C and S could probably be accomodated but I've no idea what a workable ratio might be.