[PeteKz]

The material properties of the steel head studs in a Porsche air-cooled engine do not change due to time, mileage, stress cycles, or heat cycles. The temperature that a steel stud would have to reach to change its material properties is far higher than it could experience in use.

IIRC, SAE Grade 8 steel bolts are heat treated around 800F. I expect the steel head studs are made to similar specs. To change the material properties of the heat treatment would require the stud to get close to that temperature, so let's be conservative and say 500F. Remember that the studs are in the airflow under the shroud. So they don't get as hot as the exhaust side of the head. Either the heads, valves, cylinders or pistons would fail and the oil would be cooked long before the studs could get to that temperature range.

If there is mechanical damage to any of the studs, including corrosion pitting, then absolutely change them. Otherwise, there is no mechanical reason not to reuse them. Follow Henry's recommendation to use the same studs all around (even though Porsche originally used Dilivar on the bottom row and steel on the top row from 78-on).