Historically, culture has proven to be very resilient. Over the course of decades, the Russian Empire attempted to assimilate many of the cultures amidst it's captured territories; however this proved ineffective, and today Russia is still one of the most multicultural countries in the world. Battlemaster cultures are crudely modelled after the likes of which the Russian Empire sought to destroy, therefore it doesn't make sense that they could be exchanged so easily, in such a short amount of time, purely through the works of a single diplomat.