Blown Cellulose in the Attic, How to?

Can anyone provide some tips for the in's and out's of re-doing blown insulation in the attic?
I have a ridge vent and soffet vents but no gable vents. I already have blown insulation but it is 20 years old, and all settled. Do I need to remove that first or can I just add over the top?
BV.
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Go over it, but there are chutes you staple to the roof deck so air can flow up from the soffits and the insulation does not block it.
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I saw those, but they don't go up very high. If I were too say put 2 feet of insulation, could I run, say, 4inch PVC down to those vents and up over the top of the insulation?
BV.
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