EASA and UK CAA regulations are quite specific. Pilot who want to fly a new (to them) aeroplane must receive either 'familiarisation' if it's a similar SEP variant (which does not need to be conducted by an instructor), or if it has a new feature (e.g. VP prop, retractable landing gear, EFIS, electric trim, autopilot etc.), they must receive 'differences training' from an instructor qualified on the aeroplane. In either case they may not fly the aeroplane as PIC unless the person giving familiarisation signs the pilots personal flying logbook.