[{_id:ObjectId("740b850f0406011059002b14"),product_id:10,product_state:"available",old_state:null,}]

db.collection.update({product_id:10,},[{"$set":{"product_state":"outOfStock","old_state":"$product_state"/** <-- Updates `old_state` key with the current value in `product_state` key*/}}])