Oracle 18c

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create table your_table (code) as
select * from table(sys.odcivarchar2list('CodeA18', 'CodeB18'))
union all
select * from table(sys.odcivarchar2list('CodeA19', 'CodeB19', 'CodeC19', 'CodeE19'));
6 rows affected

select code from your_table;

CODE

CodeA18

CodeB18

CodeA19

CodeB19

CodeC19

CodeE19

select replace(code, '19') as root,
       replace(code, '19', 20) as result
from your_table
where code like '%19';

ROOT
RESULT

CodeA
CodeA20

CodeB
CodeB20

CodeC
CodeC20

CodeE
CodeE20

select replace(code, to_char(add_months(sysdate, -12), 'RR')) as root,
       replace(code, to_char(add_months(sysdate, -12), 'RR'), to_char(sysdate, 'RR')) as result
from your_table
where code like to_char(add_months(sysdate, -12), '"%"RR');

ROOT
RESULT

CodeA
CodeA20

CodeB
CodeB20

CodeC
CodeC20

CodeE
CodeE20

insert into your_table (code)
select replace(code, to_char(add_months(sysdate, -12), 'RR'), to_char(sysdate, 'RR')) as result
from your_table
where code like to_char(add_months(sysdate, -12), '"%"RR');
4 rows affected

select code from your_table;

CODE

CodeA18

CodeB18

CodeA19

CodeB19

CodeC19

CodeE19

CodeA20

CodeB20

CodeC20

CodeE20