[{_id:1,name:"Annelise",movies:[{movieid:12,rating:3,timestamp:97830291}]},{_id:2,name:"Maria",movies:[{movieid:3,rating:3,timestamp:978301398}]},{_id:3,name:"Paul",movies:[{movieid:23,rating:4,timestamp:978302174}]},{_id:4,name:"Fred",movies:[{movieid:23,rating:4,timestamp:978302174},{moviedid:45,rating:2,timestamp:978302149}]},{_id:5,name:"Annelise",timestamp:97830291},{_id:6,name:"Maria",timestamp:978301398},{_id:7,name:"Paul",timestamp:978302174},{_id:8,name:"Fred",timestamp:978302149}]

db.collection.update({_id:{$in:[1,2,3,4]}},[{"$addFields":{"movies":{"$map":{"input":"$movies","as":"m","in":{movieid:"$$m.movieid",rating:"$$m.rating",timestamp:{"$toDate":{"$multiply":[{$toLong:"$$m.timestamp"},1000]}}}}}}}],{multi:true})