REPLACE sem distinção entre maiúsculas e minúsculas para MySQL

DELIMITER $$

DROP FUNCTION IF EXISTS `replace_ci`$$
CREATE FUNCTION `replace_ci` ( str TEXT,needle CHAR(255),str_rep CHAR(255))
    RETURNS TEXT
    DETERMINISTIC
    BEGIN
        DECLARE return_str TEXT DEFAULT '';
        DECLARE lower_str TEXT;
        DECLARE lower_needle TEXT;
        DECLARE pos INT DEFAULT 1;
        DECLARE old_pos INT DEFAULT 1;

        SELECT lower(str) INTO lower_str;
        SELECT lower(needle) INTO lower_needle;
        SELECT locate(lower_needle, lower_str, pos) INTO pos;
        WHILE pos > 0 DO
            SELECT concat(return_str, substr(str, old_pos, pos-old_pos), str_rep) INTO return_str;
            SELECT pos + char_length(needle) INTO pos;
            SELECT pos INTO old_pos;
            SELECT locate(lower_needle, lower_str, pos) INTO pos;
        END WHILE;
        SELECT concat(return_str, substr(str, old_pos, char_length(str))) INTO return_str;
        RETURN return_str;
END$$

DELIMITER ;

$sql = "
    UPDATE timeline 
    SET message = replace_ci(message,'".$targetuserold."', '".$targetusernew."')
";