These roles did change to some extent. Nobles continued to be extremely important in the military and the government, but the exact nature of their importance changed. In addition, they became less important economically.
In the Middle Ages, nobles dominated society. They owned their fiefs, which were the major source of wealth. They led their own little armies and therefore had power bases separate from that of the central government. They also had political control over their own territories rather than simply running those territories for the central government.
In the Renaissance, central governments became more important. Wealth started to come from trade rather than from the land. The central governments taxed this and used the money to fund their own armies. They no longer had to rely on the nobles to provide economic and military support. But the nobles kept their importance. They became the government administrators and the officer corps of the new system. This allowed them to keep a great deal of power even as they lost their traditional importance.