How to update collection of supporting documents using MongoDB and C # drivers

I have a collection that contains documents of this form:

{
    "_id" : ObjectId("50f81542a63cfa27fca7df43"),
    "_t" : "ClockRecord",
    "ClockName" : "AAA-TEST123-002",
    "IngestionDateTime" : ISODate("2013-01-17T15:14:10.757Z"),
    "FilesList" : 
    [
        {
            "FileName" : "AAA-TEST123-002.mpg",
            "FileStatus" : "Arriving",
        }, 
        {
            "FileName" : "AAA-TEST123-002.aiff",
            "FileStatus" : "Arriving",
        }
    ]
 }

      

        
        
        
      

    

which use these 2 types:

public class ClockRecord : IDatabaseRecord
{
    public ClockRecord()
    {
        FilesList = new List<ClockRecordFile>();
    }

    public string ClockName { get; set; }

    public DateTime IngestionDateTime { get; set; }        

    public List<ClockRecordFile> FilesList { get; set; }

    public BsonObjectId _id { get; set; }
}

public class ClockRecordFile
{
     public string FileName { get; set; }

     public string FileStatus { get; set; }
}

      

        
        
        
      

    

I have read many answers on Stack and mongodb sites and this seems to be the correct technique:

var collection = MongoDatabase.GetCollection<ClockRecord>("Clocks");
var update = Update.Set("FilesList.FileStatus", "TEST");
var result = collection.Update(
        Query.And(
                   Query.EQ("_id", clockDocumentID),
                   Query.ElemMatch("FilesList",Query.EQ("FileName","AAA-TEST123-002.mpg"))
                 ),  
                 update, UpdateFlags.Upsert
                              );

      

        
        
        
      

    

but nothing happens. How to update FileStatus in subdocu

My ultimate goal is to replace the additional FilesList with another subdocument, so if you know how to do that, that would be a bonus.