I think I understand where you are coming from. I think something I failed to express is that I was speaking of conquest in the context of it being a source of societal change (as related to revolutions). I agree conquest between independent, but culturally similar kingdoms, could lead to a change of rulership without any cultural impact on the population at large. And so a "domestic" revolt conducted by only a small part of the population could potentially be more "invasive" and "disruptive" than a technically "foreign" conquest.
But I think there are plenty of examples of attempted cultural change in medieval europe's history, Umayyad conquest of Hispania (and consequentially the Reconquista), Byzantines and Lombards disputing the italian peninsula, the crusader kingdoms, etc. If you're willing to look post-renaissance then you could add european colonization of Africa, India, the Americas, etc.
In the context of the game, I think the conquest of Rastedel by the Twins is much more comparable to the more invasive conquests, rather than your trivial neighbouring german kingdoms affair. So I think it will be a conquest that will be source of deep societal changes, akin to a revolution in that sense, and while some of those changes can potentially be ones the people of Rastedel could welcome as positive (like more social mobility), it is also very likely that the conquerors will trample on many other aspects of Rastedel's culture that the local population will not appreciate, in particular the ones associated with Solansia, which to me is the most damaging way of causing societal changes to a people, forced by outsiders with a very different world vision.
