This is not an answer you will like.
In the eyes of the PO, this is all you fault and you have no recourse. This is the type of problem that can be fixed fairly easily, but only if you count with the good will of all parts involved. Make sure you realize you are asking for favors, and act super nice. Even if in the end, you manage to prevail by force (unlikely), they can drag this forever and make your life very unhappy.
Yes, the PO can do the changes if both parts agree to it. There is no specific procedure on paper, but it is fairly simple and they will do it. If they feel like it.
You are really racing the clock here. Your cost of getting a new box is the pain of telling everyone your new address. Soon, the cost of the other guy giving back the box to you will be the same and he will have no reason to suffer for you.
Send this guy a letter explaining what’s going on (you know the address). Don’t expect the PO to help you contacting him, of course. It is a privacy issue. Remember that depending on this guy’s diligence in informing correspondents of his new address, you might be asking for a lot.
Meanwhile, get another box and make a temporary change of address. Do this now before they start returning your mail. Talk to the postmaster and explain the circumstances of the change of address. Since it is a change within the same PO, it will be fast and painless. Don’t be surprised to miss some magazines and catalogs, though. They are not supposed to be forwarded and even if they are trying to help you, some might slip through.
If all fails, you already have another box. Remember to make another change of address, this time permanent. Otherwise the temporary one will expire and your mail will begin to be returned.
Best of lucks.
Sapo, married to a letter carrier and former CFS employee (the ones that do change of addresses). Longtime USPS TPMH (owned a UPS store that received mail and rented mailboxes under USPS supervision)