Oracle 11g Release 2

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CREATE TABLE TEST (
    ID NUMBER(2),
    VAL VARCHAR2(100),
    constraint unq_test_id unique(id)
);
insert into   test values (1,2);
1 rows affected

insert into   test values (1,2);
ORA-00001: unique constraint (FIDDLE_RTDYFXNLEUALDMKSUQPI.UNQ_TEST_ID) violated

alter table test modify (id number(2))
-- still fails
insert into   test values (1,2);
ORA-00001: unique constraint (FIDDLE_RTDYFXNLEUALDMKSUQPI.UNQ_TEST_ID) violated

alter table test drop constraint unq_test_id
-- now it works
insert into   test values (1,2);
1 rows affected