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If there isn't anything wrong with the old other than it has settled, you can blow more insulation right on top. If it is at least up to the level of the ceiling/attic floor joists, you can install rolled batts right over the old as well. You want to get the old out of there if it is wet or has animal droppings in it.
Make sure you install the new correctly...don't block the eave ventilation.
Take a look at Olympia's posting
Air sealing attic top plates and above steps and soffit is the one you need to see.