Sorry Tom, Your "expert" instructor has been wrong before. After you
asked I did a bit more research and found the following on the AOPA
Sport Pilot FAQ.

Do I need to perform a check ride to get an additional category and/or
class rating?
************************************************** *************************************************
No, a check ride is not required. Rather, you will need to follow the
provisions of 14 CFR 61.321, which requires the following:

Receive a logbook endorsement for meeting aeronautical knowledge and
flight proficiency requirements.
Complete a proficiency check from an authorized instructor other than
the instructor who trained you on the aeronautical knowledge and areas
of operation specified.
Complete an FAA Form 8710-11, Airmen Certificate and/or Rating
Application, and present it to the instructor who conducted the
proficiency check.
Receive a logbook endorsement from the instructor who conducted the
proficiency check certifying that you are proficient in the applicable
knowledge and areas of operation and that you are authorized for the
additional category and class of light sport aircraft.
************************************************** ************************************************
This is also referenced in the Sport Pilot PTS.

Since a Glider Pilot already qualifies for Sport Pilot Privileges, It
appears to be true that you there is no written or Check ride required
to be done by the FAA or a Designee. Instead you take a "proficiency
check" administered by a flight instructor.


Brian
CFIIG/ASEL