SQL Server 2012

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create table tablename (id int, transactionno int);
insert into tablename values(1000,10000),(1001,1234),(1002,20000),(1003,50000)
4 rows affected

with cte as (
  select transactionno from tablename where id = 1000
)
update tablename
set transactionno = (select transactionno from cte) + id - 1000
where id > 1000 and id <= 2000
3 rows affected

select * from tablename


id
transactionno


1000
10000

1001
10001

1002
10002

1003
10003