Can I change the input voltage for this temperature-controlled fan circuit:?
Some of this is answered by
Can this temperature-controlled fan circuit be modified for different input voltage?

In short, even with correctly rated components for the higher voltage, (are those thermistors correctly rated?) it will probably require some adjustment to work as intended.

PTC will help to increase the sensitivity to temperature, which would otherwise be quite low.

L1 and C3 will provide some attenuation of RF noise generated by the circuit.