I would not do all that work. Without light the grass under the burm
will die, so why kill your back too? If you plan to purchase top soil
be worried about the weeds you are getting for free not the grass
underneath. What I'd do is place your top soil where you want it right
on top of the grass. Before mulching sprinlkle a product named
"Snapshot" or "Preen" this is a pre-emergent and will keep any seeds
from germinating.