Is it possible to change from a member of the Dawnguard, to a Vampire Lord or vice versa (permanently)

I activated the Dawnguard DLC approximately three days after it's release/inception, in late June, and commenced actively playing yesterday: July fifteenth, though I had already spoke with Isran, leader of the Dawnguard, I completed what I recollect to be the first quest of Dawnguard yesterday, I recollect the sole objective simply as: 'find out what the Vampires are seeking', I proceeded to do so, rescuing Serana & returning her safely to Lord Harkon, her Father. Once acquainted with the Vampires, I decided to become one myself, accepting Lord Harkon's offer.

Note: the above information is merely a scene setter, an introduction, and is ultimately unnecessary

I am currently located in what I recollect to oblivion, in which I have located Serana's mother.
I have, after much deliberation come to the conclusion that I am dissatisfied with Vampirism, and though I avidly enjoy being able to reek the plentiful benefits, I now sought to align myself with the Dawnguard, and wonder whether it is possible. Perhaps there is some path to restarting the quest itself as commencing a new game entirely seems somewhat far-fetched, and perhaps unnecessary even given my dissatisfaction.

I have however, heard of a cure, and ponder as to whether it is possible to attempt assassination on either Lord Harkon, or a fellow Lord Vampire. While I realise this shall most likely not result in their actual death, I inquire as to whether it shall have the same effect as the attack on a member of the Dark Brotherhood, and immediately initiate the quest against the band.