[{"_id":"1","history":[{"date":1674926893449,"name":"Hello"},{"date":1631548766655,"name":"Super"},{"date":1631548766655,"name":"c","dateTime":"2022-01-01"}]},{"_id":"2","history":[{"date":1674926893449,"name":"a","dateTime":null},{"date":1631548766655,"name":"b","dateTime":null}]}]

db.collection.aggregate([{"$match":{history:{$elemMatch:{dateTime:{$exists:false}}}}},{"$addFields":{"history":{$map:{input:"$history",as:"h",in:{$cond:{if:{$and:[{$gt:["$$h.date",0]},{"$eq":[{$ifNull:["$$h.dateTime",true]},true]}]},then:{$mergeObjects:["$$h",{"dateTime":{$toDate:"$$h.date"}}]},else:"$$h"}}}}}}])