DPD has actually been designed to handle most pre-order scenarios. There are 3 basic steps to selling a pre-order with DPD:
1. Create your product in DPD with the pre-order price. Instead of attaching the actual file to the product, instead set a delivery message or attach a PDF thanking them for the preorder and whatever details you want to share with pre-order customers.
Creating a product in DPD:
2. When you're ready to release the product, attach the finished product file to the product and raise the price to regular / change the name as you see fit
3. Once you've attached the regular release file to the product, send a product update from the product's details page.
Sending a product update:
The product update will send all previous buyers of the product (when it was in pre-order) the new, updated file you attached in #2 above. All pre-order buyers will get the update email and it will contain a link to download the product file.