[{"system":"AA","value":{"amount":10,"field1":"1","field2":"2","field3":"3"},"amount":{"amount":5,"field1":"9","field2":"8","field3":"7"}},{"system":"BB","value":{"amount":23,"field1":"10","field2":"20","field3":"30"},"amount":{"amount":8,"field1":"99","field2":"88","field3":"77"}}]

db.collection.aggregate([{"$project":{"tmp":{$cond:[{$eq:["$system","AA"]},[["amount",{"$multiply":["$value.amount",2.0]}],["field1","$value.field1"],["field2","$value.field2"],["field3","$value.field3"]],[["amount",{"$multiply":["$amount.amount",-2.0]}],["field1","$amount.field3"],["field2","$amount.field2"],["field3","$amount.field1"]]]}}},{$addFields:{tmp2:{"$arrayToObject":"$tmp"}}},{"$replaceWith":"$tmp2"}])