create or replace procedure altermytable is 
begin
  
  alter table mytable disable constraint mytable_fk;

end;

SQL> CREATE OR REPLACE PROCEDURE proc_modify
  2     IS
  3     cur_hdl         INTEGER;
  4     stmt_str        VARCHAR2(200);
  5     rows_processed  BINARY_INTEGER;
  6  BEGIN
  7     stmt_str := 'ALTER TABLE testing MODIFY a NUMBER';
  8     -- open cursor
  9     cur_hdl := dbms_sql.open_cursor;
 10     -- parse cursor
 11     dbms_sql.parse(cur_hdl, stmt_str,
 12        dbms_sql.native);
 13      -- execute cursor
 14      rows_processed :=
 15      dbms_sql.execute(cur_hdl);
 16      -- close cursor
 17      dbms_sql.close_cursor(cur_hdl);
 18  END;
 19  /

Procedure created.

SQL> desc testing
 Name                            Null?    Type
 ------------------------------- -------- ----
 A                                        VARCHAR2(32)

SQL> EXEC proc_modify

PL/SQL procedure successfully completed.

SQL> desc testing
 Name                            Null?    Type
 ------------------------------- -------- ----
 A                                        NUMBER

SQL> create table z_tmp (id number(10));

Table created.

SQL> edit
Wrote file afiedt.buf

  1  begin
  2   execute immediate 'alter table z_tmp modify (id number(11))';
  3* end;
SQL> /

PL/SQL procedure successfully completed.

SQL> desc z_tmp;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                                 NUMBER(11)

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
PL/SQL Release 8.1.7.0.0 - Production
CORE    8.1.7.0.0       Production
TNS for Linux: Version 8.1.7.0.0 - Development
NLSRTL Version 3.4.1.0.0 - Production