Oracle Function Based Index: Modify a deterministic function?

I'm using an own DETERMINISTIC function for a function based index on one of my tables. What will happen if I modify the PL/SQL-Code of this function? (located in an package)

Will the Index invalides? Do I have to rebuild it (manually)?

Thanks!

Edit:

Here is my example script, which I used for testing now.

CREATE OR REPLACE FUNCTION func_test(v IN NUMBER) RETURN VARCHAR deterministic IS
BEGIN RETURN 'lol' || To_Char(v); END;
/

CREATE TABLE tab_test (id NUMBER(20,0)  NOT NULL);
INSERT INTO tab_test VALUES (1);
INSERT INTO tab_test VALUES (2);
INSERT INTO tab_test VALUES (3);

CREATE INDEX idx_test ON tab_test (func_test(id));

SELECT id, func_test(id) FROM tab_test WHERE func_test(id) = func_test(2);

  --1 first records fetched in 0 ms
  --
  --| 2 | lol2 |


--Change Function
CREATE OR REPLACE FUNCTION func_test(v IN NUMBER) RETURN VARCHAR deterministic IS
BEGIN RETURN 'rofl' || To_Char(v); END;
/

SELECT id, func_test(id) FROM tab_test WHERE func_test(id) = func_test(2);

  -- 0 first records fetched in 0 ms

ALTER INDEX idx_test rebuild;

SELECT id, func_test(id) FROM tab_test WHERE func_test(id) = func_test(2);

  --1 first records fetched in 0 ms
  --
  --| 2 | rofl2 |

DROP TABLE tab_test;
DROP FUNCTION func_test;