Let's be clear, in answer to mapienja's question:
NO, there is NO change to transponder requirements in Class D from April, and NO, there is absolutely NO need to contact the controlling authority of a Class D zone by phone or any other means before flight in order to forewarn them of your non-transponder transit.
Don't make it so bl**ding complicated!
NS
aka regular operator of a non-transponding aircraft in a Class D zone