create table t ( c varchar2(32767 byte) not null)

Insert all 1's followed by a 2 at the end

insert into t(c) values (lpad('2',32767,'1'))

select substr(c,32766), dump(substr(c,32766),16) from t

Replace the last 2 by 34

update t set c = replace(c,'2','34')

Two disappeared

select substr(c,32766), dump(substr(c,32766),16) from t

drop table t