What are you talking about not possible? If you got it off marketplace than you can always redownload it. Simply delete and then go back on marketplace and select "Redownload"
Or if you got a physical copy then delete it and install to hard drive again.
Easy.
1. Dial-Up. I got lucky today and was able to take my Hard-Drive to a friends house after a funeral and he let me update all my stuff and get Dawnguard... (Which truly didn't work. Everything is working fine except Dawnguard and Xbox Dashboard. When I got back home, my Xbox went back to an older version of the Dashboard which didn't had an avatar but then got a small update from Skyrim-CD)
2. Redeem Code. I bought a Redeem Code for Dawnguard which can only be used once, if I'm correct unless Dawnguard becomes 'unlocked' for me when I used it.. Then in which case, I should be able to re-get it IF I get lucky again...
Physical Copy as in CD or something else?