[{_id:ObjectId("637f1128d8298d42bae0d4fc"),name:"Iska Paphat",age:8,cat:{name:"Malone Poppelhoffen",age:7}},{_id:ObjectId("637f1128d8298d42bae0d4fd"),name:"Elbow Frank",age:67},{_id:ObjectId("637f1128d8298d42bae0d4fe"),name:"Frank Frank",age:49,cat:{name:"Pirate Yolanda",age:4}},{_id:ObjectId("637f1128d8298d42bae0d4ff"),name:"Fluffy Yolanda",age:66},]

db.collection.update({},[{$set:{age:{$sum:["$age",1]},cat:{$cond:[{$eq:["$cat",undefined]},"$$REMOVE",{$mergeObjects:["$cat",{age:{$sum:["$cat.age",1]}}]}]}}},],{multi:true})