The only reference I can find in Part-FCL is below. As always it's a bit vague!
The main reason that I'm looking into this is for ferry flights.
FCL.710 Class and type ratings – variants
Regulation (EU) 2019/1747
(a) Pilots shall complete differences training or familiarisation in order to extend their privileges to another variant of aircraft within one class or type rating. In the case of variants within a class or type rating, the differences training or familiarisation shall include the relevant elements defined in the OSD, where applicable.
(b) The differences training shall be conducted at any of the following:
(1) an ATO;
(2) a DTO in the case of aircraft referred to in points (a)(1)(c) and (a)(2)(c) of point DTO.GEN.110 of Annex VIII;
(3) an AOC holder having an approved differences training programme for the relevant class or type.
(c) Notwithstanding the requirement in point (b), differences training for TMG, single-engine piston (SEP), single- engine turbine (SET) and multi-engine piston (MEP) aeroplanes may be conducted by an appropriately qualified instructor unless otherwise provided in the OSD.
(d) If pilots have not flown the variant within 2 years following the training listed in point (b), a further differences training or a proficiency check in that variant shall be completed, except for types or variants within the SEP and TMG class ratings.
(e) The differences training or the proficiency check in that variant shall be entered in the pilots’ logbook or equivalent record and signed by the instructor or examiner as appropriate.
GM1 FCL.710 Class and type ratings – variants
ED Decision 2011/016/R
DIFFERENCES AND FAMILIARISATION TRAINING
(a) Differences training requires the acquisition of additional knowledge and training on an appropriate training device or the aircraft.
(b) Familiarisation training requires the acquisition of additional knowledge.