मर्ज का उपयोग करने का प्रयास करें।
merge into old_customer_source t1
using (select t2.custid, t2.firstname, t2.lastname
from new_customer_source t2
) t2
on (t1.custid = t2.custid)
when matched then
update set t1.firstname = t2.firstname,
t1.lastname = t2.lastname
;