Previously purchased Scoops themes can be re-download from the app free of charge. This requires that you are using the same iTunes account that you originally used for purchase. After you confirm the purchase and enter your password it will tell you that you've purchased this item before and that it will be downloaded again for free.
If you still have the old device with Scoops installed you can transfer your purchase in iTunes. Please see this article for more info: http://support.apple.com/kb/HT1848