[{"key1":"1","key2":"2"},{"key1":"3","key2":"4"}]

db.collection.aggregate([{"$addFields":{"array":{"$map":{"input":{"$objectToArray":"$$ROOT"},"as":"item","in":{k:"$$item.k",v:{"$convert":{"input":"$$item.v","to":"int","onError":"$$item.v","onNull":"$$item.c"}}}}}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":"$array"}}},{"$merge":{"into":"collection","on":"_id","whenMatched":"replace",}}])