Re: New grass over existing sod
You can bury the existing grass without problems. Anything over 3 to 4 inches in depth will smother out what's there as well as prevent existing weed seed from germinating.
Finishing your topsoil with a layer of sod will add an additional barrier against weeds and pre-existing grass coming up, as well as give you an "instant" lawn that you'll be able to enjoy this summer, rather than nursing seed, which will prevent use of the yard for most of the summer.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!