Database
[{name:"Annie",words:[{word:"bunny",scoreHistory:[4],score:5},{word:"pink",score:5}]}]
Query
db.collection.update({name:"Annie"},[{$set:{words:{$map:{input:"$words",as:"m",in:{word:"$$m.word",scoreHistory:{$concatArrays:[{$ifNull:["$$m.scoreHistory",[]]},["$$m.score"]]}}}}}}])