Database

[{"_id":0,"KeyResults":[{"Title":"Test-1","KeyResultWatchers":[{"DisplayName":"Billy CS"},{"DisplayName":"Ayşe CS"}]}]},{"_id":1,"KeyResults":[{"Title":"Test-2","KeyResultWatchers":null}]},{"_id":2,"KeyResults":[{"Title":"Test-3","KeyResultWatchers":[]}]},{"_id":3,"KeyResults":[{"Title":"Test-4","KeyResultWatchers":[{"DisplayName":"Billy CS"},{"abd":"idk"}]}]},{"_id":4,"KeyResults":[{"Title":"Test-5-0","KeyResultWatchers":[{"DisplayName":"Billy CS"},{"DisplayName":"Ayşe CS"}]},{"Title":"Test-5-1","KeyResultWatchers":[{"DisplayName":"Billy CS-1"},{"DisplayName":"Ayşe CS-1"}]}]}]

Query

db.collection.aggregate([{$match:{"KeyResults.KeyResultWatchers.DisplayName":{$exists:true}}},{"$set":{"KeyResults":{$map:{input:"$KeyResults",in:{$mergeObjects:["$$this",{KeyResultWatchers:{$map:{input:"$$this.KeyResultWatchers",as:"eachWatcher",in:{$cond:[{$not:["$$eachWatcher.DisplayName"]},"$$eachWatcher",{FullName:"$$eachWatcher.DisplayName"}]}}}}]}}}}}])

Result