Providing your app has been well designed, the biggest thing is obviously going to be your UI. Where as the iPhone is more likely to have multiple pages/tabs, iPad apps generally seem to work best when everything is accessible from one page. Take advantage of that screen-size, and in particular take advantage of popovers. For example, if your application uses a UITabBarController and has 3 tabs, consider making the first tab your main page of your iPad app, and the other 2 tabs accessible via UIPopovers. Spend some time designing your UI before jumping into the code and you'll be fine.
A secondary thing to consider is whether you want a universal app, or two separate applications. You may like to reward users who already own the iPhone version by creating a universal app (as they will get it for free), but then they may also expect their data to be synched between the iPhone & iPad. Generally, it's my opinion that apps should be universal, as this has other advantages. If you app is universal, then any reviews/sales will affect your positions/rating in both the iPhone and the iPad app store, where as if you have two apps they are completely separate.
Something else to consider is what your Base SDK is currently set to. Apps which support iPhone OS 2.X can't be upgraded to iPad apps, so if your app is currently supporting 2.X you will possibly cut out some users with an upgrade.
Other than that, you might want to consider making a simple test iPad app first up, just to get a feel for the new controls. You'll then have a much better idea of how they can be effectively used in your app.