SQL & PL/SQL :: Modification Of Existing Check Constraint

How can I modify the check constraint without dropping the already existing check constraint.

Example: alter table table_1 add constraint const_chk check (ATTRIBUTE7 in
('READ','UNREAD');

Want to add some TENTATIVE value for the already existing one. I could do that by dropping the constraint

ALTER TABLE table_1 DROP CONSTRAINT const_chk and modifying it later as
alter table table_1 add constraint const_chk check (ATTRIBUTE7 in
('READ','UNREAD','TENTATIVE');

Is it possible to do that with out dropping the constraint?