Iirc the change happens at the distance displayed. So if it says the are 800 out then they are between 800-601, once they get to 600 it will display 600 until they are 400 out.
And just to be clear they are not 1000k, or 800k, they are 1.0k, or 800. The k signifies 1000. The distance is yards, so a plane 1.0k above you is actually 3,000 feet above you.
Hope that helps.