These are difficult questions to answer. Is there a universality to the preservation of land and power under circumstances of military conquest in pre-industrial Europe?

The answer is an unsatisfying "probably not." Originally, this answer was going to open with a long narrative about a hypothetical landowner in the vicinity of Messina during the early stages of the Norman Conquest of Sicily in order to explain the sorts of dynamic whereby there could be significant, even majoritarian, social strata immediately below the ruling class who could preserve or even augment their power following a military conquest. Indeed, the conquest of Sicily was made much easier by the presence of a class of christian landowners who didn't exactly defect to the Normans, but rather embraced a sort of enthusiastic normalcy in the wake of the conquest.

However, the high-profile and logical counterpoint would be the "other" Norman conquest. In the wake of the Norman conquest of England, a systemic policy of subjugation was implemented which resulted in the substitution of the entire Anglo-Saxon ruling class. Of course, there is room for nuance: the process was helped by the truncation of the highest Anglo-Saxon aristocracy in a single decisive victory at Hastings (their cousins in Sicily did not have this good fortune) and the speed and deliberateness with which Anglo-Saxons were substituted with Normans below the highest offices of state can be debated. However, there is no doubt that the Norman conquest of England was done much differently than the Norman conquest of Sicily.

A conqueror would often have to be ruthless, that much is certain. A century before the Normans would set foot on both Sicily and England, the Germanic Kaiser Otto descended into Italy and violently deposed four of the seven great Margraves who governed the northern half of the country. However, a conqueror could also not risk expending time, energy, and resources to substitute every single semi-prominent individual in his newly conquered territory. Otto thus dispatched of the remaining three Margraves not by force of arms, but by appointing an Imperial Vicar who took over many of their administrative and political functions. This not only pushed the remaining margraves into irrelevance, but Otto's new social order allowed for the emergence of a larger class of middling landowners and aristocrats filling the space vacated by the old margraves. Thus among this segment of the "conquered" people there emerged a prosperous class whose land and influence had increased!

As above, examples exist of both violent conquests and more orderly "institutional" conquests. Many included features of both. There is no overarching rule or pan-European method of operation in the neutralization of powerful people in a conquered territory. This is because power is transmitted in many ways beyond force of coercion: it is transmitted by controlling institutions, controlling people, and controlling rituals and symbols. A shrewd ruler would need to assess the best combination of factors and resources to effectively impose themselves. Thus loss of titles and property, if not loss of life, were often the consequences for members of a defeated social or political order (especially for those who had been prominent in the defense of that social order). But these were not the exclusive consequences.