Or you could just unplug the harddrive as in:
Also, you can load saves made with an earlier patch version after patching to a newer one. You just can't do the opposite, that is, save a game, find out a certain glitch you want to do was fixed or a bug introduced with the current patch is breaking something, then remove all patches and load that save. I encountered this when I copied a game save over to a flash drive so I could unplug the harddrive with Dawnguard on it and buy the alchemy lab. It wouldn't load the save, but the message it gave was familiar from some time before the DLC existed. After I thought about it and read a comment that mentioned profile, it occured to me, both my profile and patch data were also on the harddrive I unplugged. The objection wasn't to not having Dawnguard available, but to basically trying to load a save made under 1.6 into a completely unpatched Skyrim. Moved the patch data and profile and tried again, and it worked fine.
Just an alternate way to do the workaround if you don't feel like waiting for the half a gig DLC to redownload. Either way, sadly, cannot protect you from the loss of quest progress and unique new items, so the strong advice to do this thing before beginning to do the Dawnguard stuff still applies.
I hear that it also did not fix certain broken quests. At least there were memory optimizations that people are reporting seem to have decreased load time for people without kinect who could care less about that stuff.