Welcome to the DTV USA forum.
Without knowing your location, the best answer is you probably will not lose any channels. In most cases you will actually gain channels. With your location we should be able to help you maximize your DTV reception.
Find the thread with the link to TV Fool. That's the best way to test out your area and see how it will be. As the others have said, it will depend on your location. The TV Fool tool is a great way to get a rough idea of what stations you'll get.