I think that the easiest way of doing this is to alter the
original database  itself to record  a timestamp on each record in each
table, identifying  the last time that record was updated  (best done
with a automated Trigger).

Once this has been achieved then a timestamp could also be included
in the XL identifying the same thing.

Then when it comes to updating, you only need to import records which
have been updated more recently than time X.