It twas Uriel, page 470 hard cover copy, chapter fifty one.
Well, there you go. There's no fear of paradox (which Vadderung claimed to be over-rated anyway). Time travel exists within the individual with no direct contact to the outside world. The only affectual (sic?) design is insight and perspective.
If you're "future" Harry and you want to change something instrumental that happened in the past....Good luck, because inertia (Law of Conservation of History) will likely be too strong to overcome. However, if you wish to prepare for some unknown event yet to be accomplished, your future self can come back to your past self (via your subconscience) and push you in the desired direction to secure the knowledge your future self needs (but doesn't yet have). It's not about changing a particular event. It's about gaining the knowledge to better prepare for when that event is to occur.
As Vadderung said. To rewrite history it's easier to change the future (paraphrase).
If you haven't grasped onto this yet...try this pseudo-scenario:
Future Harry learns that "So-and-So" is behind an attempt to blow up Demonreach. However, he's never even heard of this "so-and-so." So future Harry time travels (and since past, present, and future all exist in tandem...he side steps) to past Harry's subconscience and tells him to learn all about "So-and-so." When Future Harry returns to his reality...wow...look at all that needed insight.