EASA explains clearly what it means by "variant" in the list of type and class ratings here ...
http://easa.europa.eu/system/files/d..._List_Acft.pdf
and in the explanation for that list here ...
http://easa.europa.eu/system/files/d...20T_R_List.pdf
within the SEP(Land) class rating there are 8 "variants" listed in column 2 of the list, these are ...
SEP(Land)
SEP(Land) with VP prop
SEP(Land) with retractable landing gear
SEP(Land) with turbo/superchanrged engine
SEP(Land) with cabin pressurisatoin
SEP(Land) with tailwheel
SEP(Land) with EFIS
SEP(Land) with Single lever power control
If you are changing from one of these variants to another you need differences training from an Instructor. If you are changing aeroplane types within the same variant you need "familiarization training" which means a self study of the books and any other relevant materials you consider necessary. This is set out in the Guidance Material to FCL.135 as follows ...
GM1 FCL.135.A; FCL.135.H
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.