[{"id":"abc","name":[{"first_name":"abc","last_name":"def"},{"first_name":"ghi","last_name":"mno","middlename":"xyz"}]}]

db.collection.update({"name.middlename":{$exists:true}},[{$set:{"name":{$map:{input:"$name",in:{$cond:{if:{$ne:["$$this.middlename",undefined]},then:{$mergeObjects:["$$this",{mid_name:"$$this.middlename"}]},else:"$$this"}}}}}},{$unset:"name.middlename"}],{multi:true})