That is quite easy to answer.
- for us private flyers there is no requirement to be equipped with a XPDR
- if you fly only in airspaces with no XPDR mandatory label, everything is fine and if you switch on your venerable Mode A/C, everybody is happy
- if you fly into airspaces with mandatory XPDR, such as class C, D or the more and more upcoming TMZ, then the requirement "mandatory transponder" now equals "mandatory Mode-S transponder". If you go there without Mode-S, you need to get a special clearance to enter airspace without transponder
- the last date for Mode A/C was in Spring 2008 and thereafter "to be a transponder" under EU regulations, it has to be Mode-S