If you can predict the name of the converted file, then the best approach is to modify the constructor of the Video class (in the business layer). Specifically, notice this line in the constructor:

this.Original = DisplayObject.CreateInstance(this, originalFilename, originalWidth, originalHeight, DisplayObjectType.Original, new NullObjects.NullDisplayObjectCreator());

Be sure the variable originalFilename contains the name of the converted video file (e.g. myvideo.flv). Also update the originalWidth and originalHeight to the correct width and height values (in pixels).

This approach assumes the converted file is in the same directory as the original. The save algorithm *might* throw an exception or behave unexpectedly if the converted file does not exist when you try to save it. You'll have to test it.