EASA B2 mechanic for transponder/radio installation near Berlin, Germany

I believe the “recent changes” refer to CS-STAN – a set of “standard changes” that can be carried out on aircraft meeting specific criteria.

CS-SC001a covers VHF voice communications equipment.
CS-SC002a covers mode-S elementary surveillance equipment

I think you would need a EASA Part M/145 for installing a Transponder. Maybe even a new Radio.

Unfortunately, it doesnt work like on FAA registered airplanes, where a IA can sign of the installation.

However there are some changes recently, so depending on aircraft weight, it might be possible. BWTHDIK