I cannot reproduce, works as supposed for me...What version are you on ?

  1  create table t (x number, y char(10))
  2  partition by range (x) (
  3  partition p1 values less than (100),
  4  partition p2 values less than (maxvalue)
  5* )
test@CENTRAL> /

Table created.

test@CENTRAL> select table_name, row_movement from user_tables where table_name='T';

TABLE_NAME                     ROW_MOVE
------------------------------ --------
T                              DISABLED

test@CENTRAL>
test@CENTRAL> insert into t values (10,'a');

1 row created.

test@CENTRAL> commit;

Commit complete.

test@CENTRAL> update t set x = 110 ;
update t set x = 110
       *
ERROR at line 1:
ORA-14402: updating partition key column would cause a partition change


test@CENTRAL> alter table t enable row movement;

Table altered.

test@CENTRAL> update t set x = 110 ;

1 row updated.

test@CENTRAL> commit;

Commit complete.

test@CENTRAL> alter table t disable row movement;

Table altered.

test@CENTRAL> select table_name, row_movement from user_tables where table_name='T';

TABLE_NAME                     ROW_MOVE
------------------------------ --------
T                              DISABLED

test@CENTRAL> update t set x = 10; 
update t set x = 10
       *
ERROR at line 1:
ORA-14402: updating partition key column would cause a partition change


test@CENTRAL>


Stefan