For the exams you are credited for the common subjects but will have to do the sailplane specific exams in principles of flight, operational procedures, flight performance and planning, aircraft general knowledge & navigation.
You get up to 50% off the 15 hours minimum training for an LAPL(S) or up to 7 hours off for an SPL.
Not sure about the rest of EASA-land but in the UK the only route to an EASA sailplane licence is grandfathering through BGA qualifications. If your CPL is UK CAA issued you will have to get the BGA bronze badge + xc endorsement & then apply to the CAA for a sailplane licence.