Keep in mind if you want to start over with the DLC's already installed, you will actually get the Dragonborn Quest again if your reach level 5.
So it makes no sense starting again if you leave your DLC's installed. (considering you will play withoit DLC's)
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But you can do this: put those saves on a memory stick and keep that stick safe.
After that delete your DLC's and those saves (related to Dragonborn and Dawnguard) and clean your cache.
Then begin from scratch with a new charakter.
If you want to continue with your older saves again, re-download your DLC's again !!!!!
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But if you want to start from scratch and get the Dragonborn and Dawnguard quest again, if you reach a certain level, you can leave your other saves without problems.