[{test:1,"accountCreation":[{"startDateTime":"","endDateTime":"","isDone":"Yes",another:1},{"startDateTime":"","endDateTime":"","isDone":"Yes",another:2},{"startDateTime":"","endDateTime":"",another:3}]},{test2:2,test3:3}]

db.collection.update({"accountCreation":{$exists:true}},[{"$addFields":{accountCreation:{"$map":{"input":"$accountCreation","as":"ac","in":{$mergeObjects:["$$ac",{"done":"$$ac.isDone"}]}}}}}],{multi:true})