Upgrade Apps from iPod to iPad
It depends:


There are a few universal apps (the ones that will have a + icon in their purchase area), which means that they run in a native UI on both devices. Many developers have added this to their apps for free to the user. In this case, you probably even have the updated version already, and it will work just right on your iPad. Some examples may be like Instapaper, NewsRack, 2Do, The Incident, Infinity Blade, etc. 
Other apps differentiate between the two versions, usually a normal version and what many call an HD version. Since this is a different app, they usually are priced differently and can not be updated between versions. An example would be Flight Control or Angry Birds, both have a different iPhone and iPad app, where you would have to buy both.


There is no way currently to upgrade to the better version for a discount between devices. So to answer your question, you would have to rebuy anything you want on your iPad (that's specifically made for iPad) that wasn't provided for free from the developer in an upgrade to the app (where the developer made the app universal with an update).