If you haven’t stored the old tires properly, their safety is v-e-r-y questionable, although a knowledgeable tire technician may be able to inspect them and say they may be OK. Hybrid drivers are too valuable to wind up having an accident with bad tires.
Another consideration: Driveability.
If your '06 snow tires were “energy-saving” tires you have the equivalent of a 2006 cell phone. The standard energy-saving tires on our '06 Escape hybrid were the worst tires we could remember – hydroplaning, poor handling, poor wet braking, etc. When we bought newer tires we felt we had a brand-new car. New eco-tires may be better (a site like tirerack might have some tests).
Yes, there is a calculator. Tirerack.com (or a similar site) can give rpm’s per mile for both sets of tire sizes (you’ll need to key in a tire size, then select any tire and look at the specs for that tire to find this), which you can use to estimate actual mileage changes, as well as tire diameter, which affects ground clearance. Probably won’t be a major change.
Good luck and thanks for buying a Prius (always nice when “somebody else” helps save the planet), but I agree with VDC that money isn’t the big issue here, especially with a new car.