SQL> create table  t1
  2  (a varchar2(10),
  3  b varchar2(20))
  4  tablespace ts;


Table created.


SQL> insert into t1
  2  values(1,'a');


1 row created.


SQL> insert into t1
  2  values(2,'b');


1 row created.


SQL> commit;


Commit complete.


SQL> alter tablespace ts read only;


Tablespace altered.




SQL>  alter table t1 add (c varchar2(20));


Table altered.


SQL> select *
  2  from t1;


A          B                    C
---------- -------------------- --------------------
1          a
2          b


SQL> alter tablespace ts read write;


Tablespace altered.


SQL>