No. If you delete the app you will loose the data in the app. The data is still on the website. If you purchase the two-way-sync, you can get the data information, from the website, back to you iPhone App.
If you don't delete the app, you probably don't have anything to worry about.
If your previous iPhone was correctly backed up, then you should be able to restore the apps and data to the new iPhone and loose nothing in the transformation.