MySQL 5.7

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DROP FUNCTION IF EXISTS rnd_str;

CREATE FUNCTION rnd_str(
    v_chars TEXT
)
RETURNS TEXT
NOT DETERMINISTIC -- multiple RAND()'s
NO SQL
SQL SECURITY INVOKER
COMMENT ''
BEGIN
    DECLARE v_retval         TEXT DEFAULT '';
    DECLARE u                INT UNSIGNED;
    DECLARE v_rep_str_upper  VARCHAR(35);
    DECLARE v_rep_str_lower  VARCHAR(35);
    DECLARE v_rep_num        VARCHAR(11);
    DECLARE c_char           VARBINARY(1);
    DECLARE c_new_char       VARCHAR(1);

SET v_rep_str_upper = 'ABCDEFGHIJKLMNÑOPQRSTUVWXYZ';
    SET v_rep_str_lower = 'abcdefghijklmnñopqrstuvwxyz';
    SET v_rep_num       = '1234567890';
    SET u               = 1;
    
    WHILE u <= LENGTH(v_chars)
    DO
      SET c_char = MID(v_chars, u, 1);
      
      SET c_new_char = CASE WHEN INSTR(v_rep_str_upper, c_char) > 0 THEN SUBSTRING(v_rep_str_upper, RAND()*27+1, 1)
                            WHEN INSTR(v_rep_str_lower, c_char) > 0 THEN SUBSTRING(v_rep_str_lower, RAND()*27+1, 1)
                            WHEN INSTR(v_rep_num, c_char) > 0 THEN SUBSTRING(v_rep_num, RAND()*10+1, 1)
                            ELSE c_char
                          END;

SET v_retval = CONCAT(v_retval, c_new_char);
      SET u = u + 1;
      
    END WHILE;

RETURN v_retval;
END;

SELECT nombre, rnd_str(nombre)
    FROM (SELECT 'VAN RISEL 5' AS nombre union
          SELECT 'KALENJI 58' union
          SELECT 'ASICS4' union
          SELECT 'aDIDAS') T    
;

nombre	rnd_str(nombre)
VAN RISEL 5	NDE NXDUS 3
KALENJI 58	BLWFIAE 79
ASICS4	ZPEWX1
aDIDAS	null