You need 3ds Max to change the weapons the models are using. It is not at all difficult do. Just a matter of selecting the weapon you want from one model and saving it as a .max file. Then opening the unit that you want to have the weapon, deleting the weapon you do not want, and merging in the .max file of the weapon you do want. Then unwrapping the uvw to map the texture to it.

However, 3ds Max is not free. Nor even cheap.