Thanks @MTO

Below part of your answer itself work for converting the date into the expected format without using insert command

i tried this way

~~~

create table basetable as
with test_table as (select * from basetable)
SELECT t.*, date_str,
       TO_DATE(date_str DEFAULT NULL ON CONVERSION ERROR, 'DD/MM/RR') as formatted_date
from test_table t

~~~

this worked as expected