On Tuesday, March 26, 2019 at 11:39:35 AM UTC-7, Pete Wizz wrote:
Thank you All for your answers, this gave me some clue how to solve a mystery

Pete,

My gut feeling is that adding the FES is a major modification. Just its effect on the CG alone puts it into this category. The first step is to contact your local FSDO and talk to an airframe inspector. He/she probably has the answer, but may request more information about the modification (most FAA inspectors haven't dealt with gliders before). If they want you to deal with a DAR, they will tell you that.