There really is no such thing as an 'HD' antenna. Since the digital transition most stations are using UHF frequencies, although some are still on VHF. If you have an old antenna capable of both VHF and UHF reception it may work ok, but it depends on where you are and what the signal levels are which can be determined by going to the websites that were linked above.
It is very possible that an older antenna that was acceptable for analog reception won't work properly for digital because a strong signal is required to get stable digital reception. A weaker analog signal might have a little noise or 'snow' but it would be perfectly watchable, whereas a digital signal will look perfect until a minimum threshold of signal strength below which it will then pixelate and disappear. Combine this with some stations having lower power levels post-transition and you now need a high-gain antenna to watch the same channels you used to get with rabbit ears.
You should check to see if your local stations are all on UHF or if any are on VHF, this will vary by market. Some of the so-called HD antennas being sold are UHF only which is obviously a problem if you need to see any VHF channels.
