Problem with SELECT CASE WHEN THEN, 0 result
found solution, sorry, my mistake !

SELECT i.fav_id, 
        CASE 
           WHEN i.fav_id = 2 THEN 3
           WHEN i.fav_id = 3 THEN 2
           WHEN i.fav_id = 7 THEN 8
           WHEN i.fav_id = 8 THEN 7
           ELSE i.fav_id
       END fav_id
FROM `favorites` AS i WHERE i.member = '123456'