[{_id:1,items:[{_id:1,orderNr:0,},{_id:2,orderNr:1,},{_id:3,orderNr:2,},]}]

db.collection.update({},[{$set:{item1:{$arrayElemAt:[{$filter:{input:"$items",cond:{$eq:["$$this._id",1]}}},0]},item2:{$arrayElemAt:[{$filter:{input:"$items",cond:{$eq:["$$this._id",3]}}},0]}}},{"$set":{items:{$map:{input:"$items",in:{$switch:{branches:[{case:{$eq:["$$this._id",1]},then:{$mergeObjects:["$$this",{orderNr:"$item2.orderNr"}]}},{case:{$eq:["$$this._id",3]},then:{$mergeObjects:["$$this",{orderNr:"$item1.orderNr"}]}}],default:"$$this"}}}}}},{$unset:["item1","item2"]}])