Astrotrain was designed to be Astrotrain, Sentinel was a secondary tooling. Octane and Blitzwing will look like themselves, Prime and Megatron are alternate toolings. There is no way they will just be a head and a gun change. It's not the first time they released the secondary tooling first. Not sure exactly how much will need changing, but Blitzwing won't have Megatron's chest.