So who will be able to do differences training? What about things like SET?
As far as I can see, only FIs will be able to conduct differences training as theirs are the only privileges to include instruction for class and type ratings for single-pilot, single-engine aircraft generally (i.e. without specifying "the issue, revalidation and renewal" of ratings as the CRI privileges do) and for class extensions, which I take as meaning tailwheel, VP prop, etc.
So far as SET is concerned, a CRI will be able to instruct for SET class and type ratings provided that the aircraft is not classified as a complex, high performance aeroplane (e.g. Cessna SET, etc.) Instruction for complex, high performance SET ratings (e.g. TBM700, AT802, etc.) will have to be done by a TRI(SPA). Mind you, the problem is not helped by EASA getting up to its usual tricks and throwing in new terms without defining them - I cannot immediately find a definition of 'complex, high performance aeroplane', either in the NPA or in CS Definitions. We might assume that 'complex' and 'high performance aeroplanes' are as defined as in JAR-FCL but it is only an assumption.
It is also not clear what will happen in the case of aircraft that currently hold national ratings for which there is no EASA equivalent, such as the Finist SET and DO28G-92. Since they do not qualify as Annex II aircraft they will, presumably, have to be issued with an EASA type certificate in accordance with article 5 of the Basic Regulation if current class/type ratings are to remain valid.
....are unlikely to want to go back to doing a piston FI rating
The privileges of an FI do not include instruction on complex, high performance aeroplanes, only a TRI(SPA) can do this.
I'm still working my way through the almost 4500 pages of text that comprise the CRD and so there may yet be more to come. For example, the pre-entry requirements for an FI certificate still include CPL level knowledge, although they have at least eased the requirement to actually hold a CPL. A PPL holder with a FI certificate may be remunerated but only for the provision of flight instruction for the LAPL or PPL (i.e. not for class or type ratings, differences training, etc., which must be unremunerated.)