The following article explains that, as well as a recent scam you should avoid.
By now you know that TV is changing next year. Starting Feb. 17, 2009, local TV stations will turn off their analog transmitters and go all digital.Bold emphasis mine.
If you have cable or satellite you don’t need to do anything. But if you have an analog TV and get your signal with an antenna, you’ll need a digital converter box or your TV won’t work.