[{name:"01",tags:{tag_01:{a:"a01-val",b:"b01-val"},tag_02:{a:"a02-val",b:"b02-val"}}},{name:"02",tags:{tag_03:{a:"a03-val",b:"b03-val"},tag_04:{a:"a04-val",b:"b04-val"}}}]

db.collection.update({},[{"$set":{"tags":{"$objectToArray":"$tags"}}},{"$set":{"tags":{"$map":{"input":"$tags","as":"tag","in":{k:"$$tag.k",v:{"$mergeObjects":["$$tag.v",{"tagName":"$$tag.k"}]}}}}}},{$set:{tags:{"$arrayToObject":"$tags"}}}],{multi:true})