Purchase Order Dates

Looking through purchase orders, and can see a few issues that need clarifying/potentially looked into further. P/O's currently only have a header level date, which is pulled to all the SKUs in that P/O for their expected arrival date (tooltip hover over etc). Firstly, is there any way to have this an updatable field, like alot of other fields in the P/O? 

If we get updated timeframe for orders, standard logistics and stock management procedure would be to update the ETA to these new dates. At present, once P/O has been generated, its not possible to change this date. We have some suppliers that can only provide an ETA once P/O has been placed. 

Secondly, this Data is only at a header level for the P/O and not at a line based level per SKU. We place large orders with suppliers, and due to backorders/supply chains, theres often different ETAs for each SKU. Can there be a way to implement a line level based ETAs on POs moving forward.