public class ClassExample
{
    public ObservableCollection<OriginalObjectType> OriginalObjectTypes { get; set; }

    public void ConvertNewObjectTypesToOriginal(ObservableCollection<ObjectType2> objectType2s)
    {
        ObservableCollection<OriginalObjectType> originalObjectTypesTemp = new ObservableCollection<OriginalObjectType>();

        foreach (var objectType2 in objectType2s)
        {
            var originalObjectType = new OriginalObjectType { Value = objectType2.Value };
            originalObjectTypesTemp.Add(originalObjectType);
        }

        this.OriginalObjectTypes = originalObjectTypesTemp;
    }

}

public class ClassExample
{
    public ObservableCollection<OriginalObjectType> OriginalObjectTypes { get; set; }

    public void ConvertNewObjectTypesToOriginal(ObservableCollection<ObjectType2> objectType2s)
     {
         if (this.OriginalObjectTypes == null)
             this.OriginalObjectTypes = new ObservableCollection<OriginalObjectType>();
         else
             this.OriginalObjectTypes.Clear();

         foreach (var objectType2 in objectType2s)
         {
             var originalObjectType = new OriginalObjectType { Value = objectType2.Value };
             this.OriginalObjectTypesTemp.Add(originalObjectType);
         }
     }
}

class ClassOne {
  public ClassTwo ConvertToClassTwo() {
    // do conversion
  }
}


// usage:
for (int64 index = 0; index < yourArray.Length; index++) {
  yourArray[index] = yourArray[index].ConvertToClassTwo();
}

public class ClassExample
{
    public ObservableCollection<OriginalObjectType> OriginalObjectTypes { get; set; }

    public void ConvertNewObjectTypesToOriginal(ObservableCollection<ObjectType2> objectType2s)
    {
        this.OriginalObjectTypes = new ObservableCollection<OriginalObjectType>(new List<ObjectType2>(objectType2s).ConvertAll<OriginalObjectType>(new Converter<ObjectType2, OriginalObjectType>(ConvertOT2ToOOT)));
    }

    public static OriginalObjectType ConvertOT2ToOOT(ObjectType2 objectType2)
    {
        return new OriginalObjectType { Value = objectType2.Value };
    }
}