Automatic ship date notification

I am trying to create a script that will notify customers if the ship date for an item they have ordered changes.  Basically, I created a custom column field for SOs called Date Out and a custom column field for POs called Date In.  When one of our buyers has an updated ship date from a vendor, they go to the PO and change the Date In for that line item.  I want that to then update the corresponding Date Out line on the SO. 

Here is the problem I'm running into:

When the Date In on a PO is changed (e.g. we are notified a week later that there is a delay in receiving an item), corresponding SOs will need to have the Date Out field updated and the customer will be notified.  How will this work if there are multiple SOs and POs?  As an example, customer 1 places an order for 1 widget, customer 2 places an order for 1 widget.  We then place a PO for 2 widgets.  Customer 3 places an order for 1 widget.  Customer 4 places an order for 1 widget.  We place a second