Hello Adrian,

You should be able to go to Settings->Technical->Fields and from here you can see all the fields. If you remember the name of the field you can just search for it, otherwise you can filter or group by the purchase order line model. 

When you click on the field you will see the compute field as standard and you should be able to change it from here.

To do this you do need to be in debug mode. 

I hope this helps.

Thanks,