You are correct there is no auto close because of the change orders. 

If you change the status to closed you cannot invoice or add change orders to the subcontract. 

Mid you move it to completed you can add and invoice. I like to do this if I know the work is complete so that when I look up subcontracts from the AP invoicing screen I can choose just current ones then sort by job. But is not necessary.  More of a preference thing. 

Thanks, I thought I might be missing something.