OMG i think im the first pirson to do this anyway

to refresh the dawnguard quest line delete the DLC and skyrims update go back onto skyrim update it and download the dlc again and whalla you have a new dawnguard quest line

More info on how to do this

Delete Skyrim Update & Dawnguard DLC

Play skyrim and get the update

Play skyrim and save game

Download dawnguard dlc again

DONE