I suspect the reason for the change is to allow the national CAAs some flexibility in determining precicely under what circumstances differences training is required, or when just familiarisation training will suffice.
The problem with being too specific in the regulations is that it makes it difficult to respond appropriatly to new techonologies. I think the CAA will take a practical approach in requiring differences training where significant new skills can only be properly acquired through actual flight (or simulator) training.
For example, EFIS cockpits now require diffenences training, though this was not in the original JAR document.
New engines present further issues. The original JAA document specified differences training for turbo charged engines, but they had in mind engines with waste gate / boost controls etc. The modern turbo diesels (Wilksch and Centurion) and the Rotax turbocharged engines do not have cockpit controls asociated with the turbocharger and do not require differences training (i.e familiarisation training is sufficient).
However, consider the case when the turbo diesels are used for ab initio training. It will then be possible to get a PPL or CPL without ever encountering magnetos, mixture control or carb heat control. The pilot would then require differences training before operating conventional AVGAS engines.