Hi,
looking at the documentation this is not possible and not intended. Derived columns are considered “volatile” – if you will – and must always be recomputable from scratch. You get an event with a row or a table that has had some changes occur and you need to recompute the value of your derived column. There is no indication which column or columns in the row (or rows in the table) that caused the event have changed.
Can you elaborate on why you want to use the difference / the old value instead of recomputing the derived values?