Remove old insulation or not
Usually in doing this, the new insulation is run perpendicular to the existing, but with the age of the existing, it has probably sunk to a level below the top of the joists, so you will just be creating air channels which will permit convection currents and defeat the insulation, so I would remove it. If not you could add a layer of UNFACED material in the bays to bring it flush with the top of the joists, then another layer (UNFACED also) perpendicular as above.
Considering it's age, the existing stuff is probably not worth saving, as suggested by clasact.
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