Considering the scenario in which an item number should be replaced by an alternative item number with the "nothing in warehouse" option selected:

- ATP time fence for the the Item A is set to 60 (days)
- ATP time fence for the Item B is set to 30 (days)

Now, if you create a sales order for the Item A and then adjust the sales quantity, the delivery date control based on ATP doesn't recalculate the shipping and receipt date confirmed not considering the on-hand availability or the specific ATP time fence for the alternative item number.
The design coming to delivery date control is such that SO line confirmed dates already calculated and added to the SO line are not recalculated automatically when earlier delivery is possible due to a quantity change or substitution with an alternative item. The dates may only be postponed (pushed into the future) if existing SO line confirmed dates cannot be met when changing line quantity or substituting with an alternative item.

At this moment, the work around is that every time an item is substituted, in order to be sure that the confirmed shipment date on the SO is correct, the user must delete the line and insert it again with the alternative item. Alternatively, the user must verify the date through the “Simulate delivery date” button in order to verify that the date can be anticipated.

I would suggest to find a solution for recalculating new dates every time an item number is replaced by an alternative item number on the basis of the new informations.