At EcoHome we always recommend removing the old insulation for many reasons.
Reasons for Removal when Spray Foaming
Hinders the air flow in your home
Temperature difference in the attic and rest of the house (Potential for condensation and mold growth)
Anything in the insulation (dust, dead animals, animal droppings) will be inside your building envelope, so it will be what you will breath.
Reasons for Removal when Blowing new Insulation
Exposes all possible air leaks in the attic floor, which will let us air seal efficiently
Removes everything trapped in the old insulation including: dust, dead animals, and animal droppings