Social and Economic Change, and Everything the Same
Territorial takeovers in medieval times meant the entire leadership of the subjected country simply transferred its allegiance to new rulers. In the case of Ukraine, the royalty was disbanded, but their lesser nobility kept its shape, with all its web of rank and subordination, down to the peasants and slaves.
The entire nobility had to suck up a downgrade in status, though. A Kyivan Rus’ lord was not as lordly as a Lithuanian lord.For those at the bottom, nothing much changed, except that perhaps life got tougher. The Polish plowed up the fertile grasslands of Eastern Ukraine for bread and cake. In order to plant and harvest this new gold, peasants were invited to relocate, since peasant-power was hard to find in the Ukrainian steppes. The government incentivized them by offering freedom from serfhood (the system where certain laborers belonged to certain estates and were not allowed to move). That sparked a people-movement of peasants who were longing for more freedom. When they arrived in the wheatlands, living conditions were much improved. The great Ukrainian estates thrived under their Polish landlords, and grain flowed westward.