Re: re sodding question
I've not heard of overlaying existing grass with new sod. The definite advantage is the reduction in work and expense, however IMHO, it's a bad idea. Part of the reason to till and remove the old grass is that you'll destroy the growth and root structures of the weeds, then the sod creates a pretty good blanket that will smother out most new growth, however, if that growth is already established with a good root structure, all you've done is force it to push toward the warmth and light. Weeds will likely take over much more quickly and be more problematic than having started from bare soil.
Reasons to till the soil are so that you can install or change the irrigation as necessary, regrade and amend the soil, and change the layout of the landscaping if you so desire. Tilling will aerate the soil and it will be easier for the sod to take root, it will also allow you to smooth out any dips or pockets that have formed over the years.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!