Your application may not work as expected on Tablet if
1) you have used Android APIs that have been deprecated in HoneyComb version (Android version for Tablet). This begs using new APIs, worst case redesign.
2) you have tailormade UI widget for 320x480. This could include hardcoded resolution values, small resolution images used in UI widgets. In this case you need to redesign your UI Screen and fine tune it for Tablet.
3) your workspace in Froyo /Gingerbread. You need to migrate your application workspace to HoneyComb.
4) your app has put any limitation because of hardware. This I derived from the fact that your current app has been developed for 320x480 resolution, must be a slow hardware. But Tablet are powered by 1 - 1.5 GHz processors. You may need to have a look at new set of limitation (or freedom ?). For example memory limit on application may have gone up.
In short I would suggest have a different workspace for Tablet, however if your application has classes that are independent of above stuff, then you can share your package across application. (Tablet Vs Phone App).
Shash