Adding or removing old attic insulation is necessary because of the settling that occurs over time. If you haven't had your attic insulated in a while, the first thing you'll notice is the studs in the floor. The current old insulation may still have some R-value, but this is a clear sign that additional insulation is needed in the attic.
In this case, the dilemma is whether or not to remove the old insulation from the attic. However, as we noted on our attic insulation page, old fiberglass attic tends to grow mildew and attract insects, so this isn't necessarily the case. There is no need to remove attic insulation if there is no evidence of water damage or symptoms of insect or rodent infestation in the area.
Your air registers may be covered with dust, and you should get attic insulation removal service done if you're concerned about the purity of the air there. Before placing fresh insulation in the attic, seal the floor.