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The crossbow looks the same whether the character is in attack pose or not. It doesn't "bend." If the crossbow from Heroes 7 is larger... then you can simply increase the size of the crossbow from Heroes 6 to match. What's the problem?
I'm saying that this is a simpler solution than editing a bunch of animations.
Or, as an alternative, you could take the crossbow model from Heroes 7 and use it for the model in Heroes 6.