Vlad Tzepesh said:
Most of them can be changed by conquer!
Not always. The great majority of conquerors, at all times, kept the former institutions, at least the local ones. Otherwise, they were facing the risk of rebellions.
The same is even more true in the case of DA (inheritances and others) : the new ruler sought to be accepted, at least by the more powerful notables.
That's for the legal system, currency & customs. In the majority, it's was better to keep them. As an example, look at the events for the OE, when they conquer Byzantium : historical choices (for religious matter, there) is to keep a low-profile, without changing too much.
And as for culture (I mean folk culture, legends, litterature, clothes, etc) and origins (ethnic), this can't be changed.