PO Box - name change?

I work for a European company that has changed our legal entity name (in US) for re-branding purposes. We have a PO Box for receiving accounts payable invoices and customer checks (for those unwilling to pay via ACH).

For the time being, we are receiving mail under both our old and new names. We recently had mail issued to our new name returned to sender. I didn't expect this to happen as it didn't the last time that we changed names.

I'm planning to go to the Post Office with our various legal documents referencing the name change and request that both names are accepted. Is there anything more to the process that I should expect?