The ATSC digital signal uses the existing TV band - just less of it. Many areas will lose their VHF spectrum. UHF is getting capped lower. Already dropped from Ch83 to Ch69, I think it gets trimmed to the high 50s after the switch. This all works because (theoretically) the digital signals can be spaced closer - for instance a market can have signals on CH31, CH32 & Ch33 without them interfering. Once you lose the separation issue, 2/3 of the old UHF spectrum is more than adequate. So yeah, old TV antennas should work. They may just not work well, because the ATSC signal is REALLY fragile. I've used rabbit ears to get the signal. Of course with digital either it works perfectly or utterly fails. I've lost reception from trucks going by and people walking around the room - presumably causing the kind of subtle reflections that would barely be noticable with an NTSC signal.
Zenith/LG (the patentholder) has what seems to be the holy grail of ATSC recievers - the 5th Gen chipsets, that reportedly provide really robust reception, but as far as I know, it's not available in a STB, just built into really pricey sets... and 1 PC card which has varying reports of driver support.
Europe & Asia (and some mobile digital technologies) use a more robust modulation method (COFDM vs our 8VSB) but that's less about tech than the politics of Intellectual Property rights & who makes license fees, so this probably isn't the place to rant about that. Anyway, it's done, so rather than whine, we need to focus our energies on making this pig fly.
You can keep your old set (if you want DVD quality - but not HD), you MIGHT be able to use your existing antenna, but you may need to futz - a lot! All you really need is an ATSC tuner, and what's out there so far is really unimpressive. IMHO.