You can do this several ways:
1. you can move the finished tray to the top position and leave the lid off for a few days, the worms will move down away from light and the drying bedding.
2. You can put the finished castings in a bucket and top feed and wrangle up all the stragglers that way.
3. You can harvest them (here is a video on worm harvesting: http://youtu.be/OgliNESi8jA
4. you can use them worms and all, it wont matter much.
Hope this helps. And fyi, in my experience, using a tiered system, the worms tend to stay small and not multiply too much, due it being a small space. Worms need room to grow and breed, they self regulate when the space is small. I know the manufacturers say otherwise but I just have not seen it myself and I have had a working worm factory for demonstration in my classes for 8 years. I use water troughs for my breeding bins.