SQL & PL/SQL :: Replacing A Foreign Key With Another

I have an already populated table that refers to a wrong foreign key in another table. I have table abc that has the column fk_id. this column currently refers to column x_id1 in the table abc while it should refer to column x_id2 in xyz. I am trying to replace the data but I guess I miss something in the logic or the right way of doing it:

following is my trial:

create table abc
(
pk_id number(4) primary key,
abc_name varchar2(10),
fk_id number(4)
);

create table xyz
(
x_id1 number(4),
x_id2 number(4),
x_name varchar2(10)
);

commit;
insert all
into abc values (10,'ab1',1)
into abc values (11,'ab2',2)
into abc values (12,'ab3',5)
into abc values (13,'ab4',7)
into abc values (14,'ab5',9)
into xyz values (1,1,'d1')
into xyz values (2,2,'d2')
into xyz values (5,3,'d3')
into xyz values (7,4,'d4')
into xyz values (9,5,'d5')
select * from dual;
commit;

--this following select returns 5 rows

select * from abc, xyz where abc.fk_id = xyz.x_id1;

-- the following update only updates 3 rows and sets the other 2
-- to null!

update abc set fk_id = (select x_id2 from xyz where xyz.x_id1 = abc.fk_id);
commit;

select * from abc;