Syntax:
ALTER PROCEDURE proc_name [characteristic ...]

characteristic:
    { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA }
  | SQL SECURITY { DEFINER | INVOKER }
  | COMMENT 'string'

This statement can be used to change the characteristics of a stored
procedure. More than one change may be specified in an ALTER PROCEDURE
statement. However, you cannot change the parameters or body of a
stored procedure using this statement; to make such changes, you must
drop and re-create the procedure using DROP PROCEDURE and CREATE
PROCEDURE.

You must have the ALTER ROUTINE privilege for the procedure. (That
privilege is granted automatically to the procedure creator.)

URL: http://dev.mysql.com/doc/refman/5.1/en/alter-procedure.html

