Yea dude, nice to see a new reader again! There's something incredibly exciting about being a part of something that will rival Marvel and DC in the future.
Don't worry about reading every single thing in order to catch up. If you're interested in the Valiant Next stuff (The Valiant, Ivar: Timewalker, Imperium, Divinity, Ninjak, Bloodshot: Reborn, Quantum & Woody Must Die!, Dead Drop) just start up at any of these number ones. They're all pretty freakin great, but you need to make buying The Valiant 1-3 a priority, asap.
If you care about reading the entire thing, just pick a favorite and start at #1. You don't need to know about the entire universe in order to enjoy any one book. You've already read X-O and Unity, so I'd recommend sticking with those so you can read the Armor Hunters event later. Other than that, you should check out the funny side of the universe with Quantum & Woody or Archer & Armstrong.
Yea, harashkupo has a nice reading list in his signature. Just go back a page or so.
I'm not sure what you mean by 'changes'. If you're talking about the whole Valiant Next initiative thing, here's the big shifts:
Bloodshot -> Bloodshot: Reborn by Jeff Lemire and Mico Suayan
A&A -> Ivar, Timewalker by Fred Van Lente and Clayton Henry
Harbinger -> Imperium by Joshua Dysart and Dough Braithwaite
Quantum & Woody ongoing -> Valiant-Sized One-Shot, then Q&W Must Die mini-series.
Other than that, X-O and Unity keep going uninterrupted, and Rai came bake from a break, and is going on another after April's issue. And we got some awesome mini-series in the form of Dr. Mirage, The Valiant, and Divinity.
What have you read so far?