function prepared plan

Hi everyone! I have a little problem, and was wondering if anyone could help me. 
I've created a simplified working example to show you what's going on:

//table with data
CREATE TABLE testTable (  intValue INTEGER,  textValue TEXT
);
INSERT INTO testTable VALUES (1, 'one');
INSERT INTO testTable VALUES (2, 'two');
INSERT INTO testTable VALUES (3, 'three');
INSERT INTO testTable VALUES (4, 'four');

//type used by my "test()" function to return the data
CREATE TYPE myType AS (  value TEXT
);

//function that returns a set records of type "myType"
CREATE OR REPLACE FUNCTION test(BOOLEAN) RETURNS SETOF myType AS '
DECLARE  flag ALIAS FOR $1;  mt myType%ROWTYPE;  r RECORD;
BEGIN  FOR r IN     EXECUTE testSQL(flag)  LOOP     mt.value:=CAST(r.value AS TEXT);     RETURN NEXT mt;  END LOOP;
RETURNNULL;
 
END;
' LANGUAGE 'plpgsql';


//function that dynamically generates a query for my "test()" function
CREATE OR REPLACE FUNCTION testSQL(BOOLEAN) RETURNS TEXT AS '
DECLARE  flag ALIAS FOR $1;  result TEXT;
BEGIN  IF (flag) THEN     result:=''SELECT textValue AS value FROM testTable'';  ELSE     result:=''SELECT intValue AS
valueFROM testTable'';  END IF;
 
  RETURN result;
END;
' LANGUAGE 'plpgsql';

//try the function
franco=# SELECT * FROM test(true);value
-------onetwothreefour
(4 rows)

//try it again 
franco=# SELECT * FROM test(false);
WARNING:  Error occurred while executing PL/pgSQL function test
WARNING:  line 9 at assignment
ERROR:  type of r.value doesn't match that when preparing the plan

Of course if y reconnect to the database:
franco=# SELECT * FROM test(false);value
-------1234
(4 rows)

franco=# SELECT * FROM test(true);
WARNING:  Error occurred while executing PL/pgSQL function test
WARNING:  line 9 at assignment
ERROR:  type of r.value doesn't match that when preparing the plan

does anyone know if there is any way to unprepare the plan between queries? I don't care about performance, 
so re-preparing the plan every time is ok for me.

Thanks in advance.

PS: I know the example doesn't make much sense, but in the real life case it does.