'create or replace function' no longer allows parameters

In 8.4.4 I used to be able to rename input parameters via "create or
replace function".
In 9.0 beta2 this no longer is allowed, and I get a descriptive message
informing me to use
drop function instead, but I couldn't find this documented anywhere as a
change between 8.4 and 9.0.
--------------
test=# CREATE FUNCTION test_plpgsql(IN a integer) RETURNS integer AS
test-# $BODY$
test$# BEGIN
test$# RETURN $1;
test$# END;
test$# $BODY$
test-# LANGUAGE 'plpgsql' STABLE;
CREATE FUNCTION
test=# SELECT * FROM test_plpgsql(34);
test_plpgsql
--------------
34
(1 row)

test=# CREATE OR REPLACE FUNCTION test_plpgsql(IN b integer) RETURNS
integer AS
$BODY$
BEGIN
RETURN $1;
END;
$BODY$
LANGUAGE 'plpgsql' STABLE;
CREATE FUNCTION
test=# SELECT * FROM test_plpgsql(34);
test_plpgsql
--------------
34
(1 row)

test=# \df+ test_plpgsql
List of
functions
Schema | Name | Result data type | Argument data types |
Type | Volatility | Owner | Language | Source code | Description
--------+--------------+------------------+---------------------+--------+------------+----------+----------+----------------+-------------
public | test_plpgsql | integer | b integer |
normal | stable | dgardner | plpgsql | |
: BEGIN
: RETURN $1;
: END;
:
(1 row)
-----------------------

psql (9.0beta2)
Type "help" for help.

test=# CREATE FUNCTION test_plpgsql(IN a integer) RETURNS integer AS
test-# $BODY$
test$# BEGIN
test$# RETURN $1;
test$# END;
test$# $BODY$
test-# LANGUAGE 'plpgsql' STABLE;
CREATE FUNCTION
test=# CREATE OR REPLACE FUNCTION test_plpgsql(IN b integer) RETURNS
integer AS
test-# $BODY$
test$# BEGIN
test$# RETURN $1;
test$# END;
test$# $BODY$
test-# LANGUAGE 'plpgsql' STABLE;
ERROR: cannot change name of input parameter "a"
HINT: Use DROP FUNCTION first.

--
David Gardner
Pipeline Tools Programmer
Jim Henson Creature Shop
dgardner@creatureshop.com