While I am disapointed to see a role being recast, I wouldn't mind if Zachary Levi took over. I love Chuck, and it would be interesting to see Levi in a fantasy role like this.
Preemptive strike to keep him from being The Flash?
What do you mean? Actors have played multiple comic book character movie roles before. A certain douchebag that will go unnamed was in Green Lantern, X-Men Origins: Wolverine, and Blade Trinity.
Chris Evans also played the Human Torch and Captain America, doing a much better job in the second role.