Changing Items On An Order And Auptmatically Updating Inventory

[Tom]

Using AccessXP - Access2000

In my application, when an order is entered, inventory is automatically
adjusted, a pull list is created, items are pulled to fill the order, and the
order is packaged for shippment. The order is then held pending payment and
finally shipped when payment is received. Sometimes prior to shipment the
customer calls and wants to change one or more items, wants to change the
quantity of one or more items or wants to add one or more new items to the
order. I have an order/order details form/subform for entering the changes. I
need some help on how to automatically adjust inventory for the changes.

A. Change one or more items ---
(a) The quantity for the existing item must be added back to inventory. I
presume this would be done in the AfterUpdate event of the item. When I change
the item, I now know what the new item is and what the quantity was for the old
item. How do I know what the old item was after I change the item so I can add
the quantity of the old item back to inventory?
(b) The quantity of the new item must be subtracted from inventory. I presume
this would be done in the AfterUpdate event of Quantity. The item has been
changed so I know what the new item is. The quantity has been changed so I know
the quantity of the new item. So I just subtract the new quantity from
inventory - seems straight forward ????

B. change the quantity of one or more items -----
If the new quantity is more than the old quantity, the difference must be
subtracted from inventory and
if the new quantity is less than the old quantity, the difference must be added
back to inventory. I presume this would be done in the AfterUpdate event of
Quantity. How do I know what the old quantity was after I change the quantity so
I can determine if the new quantity is more or less than the old quantity and
then be able to add back to or subtract from inventory?

C. add one or more new items to the order ---
(a) When I add a new item, how do I determine that the item is a new item and
not an existing item that is being changed where A. above would apply?
(b) When the quantity for the new item is entered, how do I determine that the
quantity is for a new item and not an existing item that is being changed where
B. above would apply?

I will appreciate all the detail anyone can give me!

Tom