Hello Tom,

Open both files. Add a third column into Old.xls: Enter into C1:
=IF(ISERROR(MATCH(A1,[New.xls]Sheet1!$A:$A,0)),B1,[New.xls]Sheet1!B1)
and copy down.

Check values in column C thoroughly. If you are satisfied with the
result, copy their values (not formulas!) into column B and delete
column C.

Regards,
Bernd