Hi Adam,
You will need to get the system updated if you want to measure your suppliers, there is no workaround for this.
There are multiple reasons why the delivery date should be updated in the system. If the system is not updated, your MRP will create replenishment orders in a non accurate way, the ATP is not accurate, you can not run a missing parts check, etc..
Now, when the Purchase Order is created and the delivery date is entered, at that moment, the system determined the statistical delivery date. When a planner updates the purchase order delivery date, this action does not modify the statistical delivery date. The statistical delivery date should only be modified if you agree with your supplier to postpone the delivery. Maybe this can be automated by a user exist or a an unknown customizing, but I don't think you can should automate this process, this should be done only if you agree with the supplier such delay.
Kind Regards,
Mariano