In the case you mention regarding the UK PPL then both ratings Group A and SLMG were in the licence and could be activated by a C of T. The PPL (Aeroplane) SLMG course was conducted by instructors who did not meet the requirements of ICAO Annex 1 so there will be no aditional ratings in the licence, so apart from the SLMG there will be nothing else to activate. Having converted many SLMG pilots to PPLs there never was a simple C of T option.