Default storage of locally managed tablespaces "created" by migration from 
dictionary managed tablespaces can still be altered. LMTs created by CREATE 
TABLESPACE EXTENT MANAGEMENT LOCAL with or without UNIFORM SIZE cannot.

On 9.0.1.4, Windows 2000

SQL> create tablespace test datafile 'f:\temp\test.dbf' size 1m default
storage (initial 8k next 8k maxextents 50) extent management dictionary;

Tablespace created.

SQL> select * from dba_tablespaces where tablespace_name = 'TEST';

TABLESPACE_NAME                BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT
MIN_EXTENTS MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS    CONTENTS  LOGGING
------------------------------ ---------- -------------- -----------
----------- ----------- ------------ ---------- --------- --------- ---------
EXTENT_MAN ALLOCATIO PLU SEGMEN
---------- --------- --- ------
TEST                                 8192          16384        8192
1          50           50          0 ONLINE    PERMANENT LOGGING
DICTIONARY USER      NO  MANUAL


SQL> exec dbms_space_admin.tablespace_migrate_to_local('TEST', 1)

PL/SQL procedure successfully completed.

SQL> select * from dba_tablespaces where tablespace_name = 'TEST';

TABLESPACE_NAME                BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT
MIN_EXTENTS MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS    CONTENTS  LOGGING
------------------------------ ---------- -------------- -----------
----------- ----------- ------------ ---------- --------- --------- ---------
EXTENT_MAN ALLOCATIO PLU SEGMEN
---------- --------- --- ------
TEST                                 8192          16384        8192
1          50           50          0 ONLINE    PERMANENT LOGGING
LOCAL      USER      NO  MANUAL


SQL> alter tablespace test default storage (initial 32k next 32k maxextents
unlimited);

Tablespace altered.

SQL> select * from dba_tablespaces where tablespace_name = 'TEST';

TABLESPACE_NAME                BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT
MIN_EXTENTS MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS    CONTENTS  LOGGING
------------------------------ ---------- -------------- -----------
----------- ----------- ------------ ---------- --------- --------- ---------
EXTENT_MAN ALLOCATIO PLU SEGMEN
---------- --------- --- ------
TEST                                 8192          32768       32768
1  2147483645           50          0 ONLINE    PERMANENT LOGGING
LOCAL      USER      NO  MANUAL


SQL> drop tablespace test including contents and datafiles;

Tablespace dropped.

SQL> create tablespace test datafile 'f:\temp\test.dbf' size 1m extent
management local;

Tablespace created.

SQL> select * from dba_tablespaces where tablespace_name = 'TEST';

TABLESPACE_NAME                BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT
MIN_EXTENTS MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS    CONTENTS  LOGGING
------------------------------ ---------- -------------- -----------
----------- ----------- ------------ ---------- --------- --------- ---------
EXTENT_MAN ALLOCATIO PLU SEGMEN
---------- --------- --- ------
TEST                                 8192          65536
1  2147483645                65536 ONLINE    PERMANENT LOGGING
LOCAL      SYSTEM    NO  MANUAL


SQL> alter tablespace test default storage (initial 32k next 32k maxextents
unlimited);
alter tablespace test default storage (initial 32k next 32k maxextents
unlimited)
*
ERROR at line 1:
ORA-25143: default storage clause is not compatible with allocation policy


SQL>


I guess if we could make extent ALLOCATION_TYPE USER, we could alter the
default storage of the tablespace. But the AUTOALLOCATE clause sets it to
SYSTEM and UNIFORM sets it to UNIFORM. Can't get it to USER.