create table tst_char(col1 char(20 byte));

insert into tst_char select trunc(dbms_random.value*power(10, 17)) || 'a' from dual;

commit

select col1, length(col1), lengthb(col1) from tst_char;

alter table tst_char modify(col1 varchar2(20 byte));

update tst_char
  set col1 = trim(col1);
select col1, length(col1), lengthb(col1) from tst_char;

drop table tst_char purge;

create table tst_char(col1 char(20 byte));

alter table tst_char
add constraint tst_char_chk1 check 
(col1 like '%a%')
enable;

insert into tst_char select trunc(dbms_random.value*power(10, 17)) || 'a' from dual;

commit;

alter table tst_char modify(col1 varchar2(20 byte));