[{_id:"61234XXX",plates:[{_id:"123","color":"orange"},{_id:"124","color":"blue"}]},{_id:"63456XXX",plates:[{_id:"321","color":"orange"},{_id:"432","color":"green"}]}]

db.collection.update({_id:"61234XXX",plates:{$exists:true}},{$set:{"plates.$[elem].plateStyle":"testValue"}},{arrayFilters:[{elem:{$ne:["$plates",[]]}}]})