Or something like this:

with Obsolete as (

select

c1,

c2

from

dbo.tblOld

except

select

c1,

c2

from

dbo.tblNew

)

update t set

obsolete = 1

from

dbo.tblOld t

where

exists(select 1 from Obsolete o where o.c1 = t.c1 and o.c2 = t.c2);