The fourth-generation iPad that Apple announced yesterday hardly renders the previous iPad obsolete. It does offer a speedier processor, faster graphics, a higher-resolution webcam (720p versus VGA resolution video), and a cellular version that can be used with Sprint's 4G data network. But in price and most other specifications, the new iPad is the same as the previous one.
The new iPad also uses the new Lightning connector that Apple introduced with the iPhone 5. In the short run, to use a Lightning connector with the 30-pin accessories designed for previous iPads, you must buy an adapter. But Lightning should be directly compatible with virtually all new iPad accessories Apple produces from now on.
If you recently bought a third-generation iPad, though, you may be wondering whether you can exchange it for the new model. First, you can return most products to Apple for an exchange or refund within 14 days of the date you received the product. Details and conditions are in Apple's return policy.
There are reports that at least one San Francisco store has extended the iPad exchange period to 30 days. Since other stores may well decide to do the same, it's worth calling the Apple store nearest you to check whether they'll make an exception for you.
Apple's return policy [Apple.com]
Peeved? Apple will exchange your 3rd-gen iPad for the newer model [Cnet]
List of Apple retail stores [Apple.com]