Well this is the Kevin Feige era of Marvel. I'm sure Levi would get locked into a contract, and if he isn't and left to do Flash, they'd kick him out the door and recast the part again for Thor 3.
All the recasts so far have made sense.
This new Frandral recast is just because the original actor is too busy on his new show. Levi was the original choice anyway.
Rhodey was recast because Terrence Howard was asking for too much money. Terrence Howard got paid more money than the star RDJ in the first IM which is ridiculous. Giving him even more money for the sequel would have been silly. I think Howard was dumb not to take a pay cut since he was going to play War Machine!
Edward Norton recast for Hulk was was because Norton was too hard to work with. Mark Ruffalo is widely considered by both fans and critics to be much better than Norton as Banner anyway. Nothing was lost.