Well, personally, I would imagine that if a purchase order was fulfilled and converted to Purchase Invoice that I wouldn’t actually consider it a purchase order anymore and wouldnt need to see it in the purchase order tab at all.
But, I feel that some people would not like that idea and would still want to see converted purchase orders still in that tab. Hence, I would simply want them to be segregated with the status showing “order received” and for the remaining to say “order outstanding”. Also, all the orders with the status “order received” should be at the bottom, separating them from the outstanding orders. This would be similar to Sales Invoices where the invoices that are paid would have a green status saying “paid” (and be at the bottom) and the ones still due would be in red saying “x days over due” (and are at the top).
But im sure you would find a great way of distinguishing the two as you have in the past with most of your other modules.