[{"_id":NumberLong(2761633),"z.z.z":"ok","test":{"a":"a","b":{"b":1,"b.b":"b"},"c":"c"}}]

db.collection.aggregate([{"$set":{"test":{"$mergeObjects":["$test",{"b":{"$setField":{"field":"b","input":"$test.b","value":"$$REMOVE"}}}]}}}])