I think that you will find that only two things are changing in March/April.
A number of exceptions to transponder regulations applicable only to gliders and SLMGs come to an end (e.g. they now need to carry a Mode S transponder above FL100). (Although I believe work is going on to allow gliders to fly above FL100 without transponders in specific areas to allow wave flying to continue.)
Light aircraft that are flying in circumstances where transponders are required have been able to continue to use Mode A/C transponders if they were fitted to an aircraft prior to a specified date (31st March 2008). This is withdrawn and a Mode S transponder must be used in those circumstances. That includes all TMZs and Class D airspace when IFR.
However, for light aircraft, gliders, microlights etc wishing to cross Class D airspace under VFR nothing will change to require a transponder if it is not required now. Nothing will change to ban the use of Mode A/C in Class D airspace if the flight is VFR.
Mode A/C transponders may still be used outside controlled airspace below FL100 at all times.
UK AIP, section GEN 1-5 Pages 14 through 17 gives full details.