Is it possible to add project accounting info on closed PO?

Hi all,

Client is performing a review of all of their PO's. They require that the project accounting information within distributions should be populated on each PO (Purcahsing Super User > Purchase Orders > Open > Shipments > Distributions > Project (tab)).

We've found a number of PO's from past few months that do not have this populated. When we go in to the area of the PO, the field is greyed out:

Wondering if anyone can advise how to get around this so that we can enter the project and task information currently missing? We've noticed a high amount of PO's without the above being populated so possible solution to apply on mass?