HDTV is now officially being broadcast to replace the old standard defiinition TV. All the old 'over the airwaves' TV channels are gone and cannot be watched anymore. All those old TV's and VCR's sitting around will no longer pick up television broadcasting. They may however be able to pick up cable channels. But, this might not be for long. Many cable companies are finding that switching everything to 'HDTV will be less expensive than maintaining the older standard definition channels as well. They will be switching over entirely to HDTV in the near future and will likely require you to acquire a new set-top box to convert their HDTV cable to play on your older TV's etc.
In some cases, it is possible to use your older TV and VCR to record or monitor HDTV signals by using the composite video output from the cable or satellite box. Simultaneously, the component video/digital audio output or HDMI output will supply HD programming to your HD screen.
HDTV broadcasting does not necessarily require you to purchase a new antenna. You may still be able to use your old one if it covers both the VHF and UHF band. Some will tell you that you need a new fangled HDTV antenna that may look entirely different from your old one. In fact, those new HDTV antennas may work worse than your existing antenna on your roof. The new HDTV signal still uses the same TV band, however, most of the local stations have been changing their channel frequency from VHF band (Channel 2 - 13) to UHF (Channel 14 - 69). This was to allow them to begin their new HDTV broadcast before they eliminated their old standard broadcast.
Also, in the old days, each TV channel number was allotted a fixed frequency by the Federal Communication Commission (FCC). Channel 5 was always at 79 MHz no matter what city or state you visited. Today, new HDTV's have to scan the available frequencies and find out what channel "name/number" was assigned to a given broadcasted frequency. This may be different from region to region. For example, if you move from Seattle to L.A., you will have to make your HDTV rescan the frequencies to find channel 5.