INSERT INTO table(id, A, B)
SELECT 
    get_next_id('id_seq', next_id, 1000),
    id, 
    CASE 
        WHEN B = 'B1' THEN 'A2'
        WHEN B = 'B2' THEN 'A3'
    END
FROM 
    table
WHERE 
    B IN ('B1', 'B2');