Akon_> create table brisime (a number not null);
Akon_> insert into brisime values (1);
1 row created.
Akon_> alter table brisime modify a null;
alter table brisime modify a not null;
If I understood your problem well, as you can see - there's no need for you to do anything with that column.
You would have to do something if you wanted to change columns datatype or decrease its size, for example. But, you didn't want to