Retain a copy of approved/signed proposals even if I unlock it.

I have to change a work order and the work order already has been approved on the customers end. And if I unlock it and then edit it, that will lose approval status for that first one. There is no documentation that they approved the first order. That's what I'm afraid of. If I unlock it, it loses status, but the question is does it save the approval status somewhere in the system so if the customer says well, I'm not gonna approve it anymore. Do I lose that approval from the original one? Is it safe somewhere so if I can go back and fight that just a question if it does happen?