The FCC has had a lot of stations changing broadcast frequencies lately. It seems to be called a 'repack' and has affected a number of stations in our area. A lot of times all you have to do is re-scan so you TV can find the stations on the new frequency. They will still show up as the same channel number.
Check out the information in this thread...
You may want to go back several pages but there is a lot of over the air information and help there.