Then it gets a bit tricky when it comes to decide if you need or not differences traiing when transitioning from say a DA40 to a clasic variable pitch prop
No, it doesn't get tricky at all. All of the required SEP differences training is listed
here. Before first flying as PIC in an SEP aeroplane that falls into any of the categories listed, differences training is mandatory (e.g. from DA40 to classic VP prop or vice versa). Once the training has been completed, it is valid for any other SEP type in the same category.
BTW It is EASA's proposal that the differences training will have to be completed if the particular aircraft category has not been flown for more than 2 years. For example, if you undertake SLPC to VP differences training and then do not fly an SLPC type for 2 years, you will need differences training to move back.
In the case of MEP aeroplanes, there is no need for similar categorisation as differences training is mandated for every new MEP type. The EASA 2 year rule will apply also to all MEP types.
Note that differences training is NOT required between petrol/diesel engined SEP aeroplanes.