I've driven up to the mountains with my summer tires, during this time of year. I start at 5,000 feet (50°) and drive up to 10,000 feet (30°).
There is a significant difference in handling, from the rubber hardening. But most high performance summer tires, perform very badly (and unsafely) below 40°. It has more to do with the temperature than the altitude.
I have driven in light snow with my summer tires, and my thought was that as long as there isn't snow on the road, the friction from the tires will keep them heated enough to provide sufficient traction, but I wouldn't risk it very long. I only did this until my winter tires arrived. In freezing weather, driving on my summer tires was very scary because there was almost no traction.
I've also found that my car doesn't run as well (overheating to the 200+ mark) at 10,00 feet, and I am guessing it's because the air is so much thinner.
