Here is my final solution:
update a
set IDnum=b.IDnum
from tempimiportable A inner join
(select * from archivetable
where IDnum in
(select max(IDnum) from archivetable
where IDnum in
(select IDnum from tempimporttable)
group by left(IDnum,7)
)
) b
on b.IDnum like a.IDnum + '%'
WHERE
*row from tempimport table = row from archive table*
to set incoming rows to the same IDnum as old rows, and then
update a
set patient_account_number = case
when len((select max(IDnum) from archive where left(IDnum,7) = left(a.IDnum,7)))= 7 then a.IDnum + 'A'
else left(a.IDnum,7) + char(ascii(right((select max(IDnum) from archive where left(IDnum,7) = left(a.IDnum,7)),1))+1)
end
from tempimporttable a
where not exists ( *select rows from archive table* )
I don't know if anyone wants to delve too far into this, but I appreciate contructive criticism...