So you've decided to Set Right What Once Went Wrong. Temporal Paradox is not a problem; once you make this change, the dark future (your old present? Whatever) will utterly change. There's a very dark possibility of The Sadistic Choice here. What if there's no such thing as In Spite of a Nail? What if the holocaust you're trying to prevent resulted in someone being born who will no longer exist if the situation of his parents getting together changes? What if there's millions of people who will be changed or erased against their will? What if this timeline had someone do a Heel Face Turn who won't in the original timeline?