Can this temperature-controlled fan circuit be modified for different input voltage?
Richman's advice is good regarding the capacitor rating. However at twice the input voltage, the caps will charge up to the trigger voltage faster. So you will need to increase - probably double - the values of C1 and C2 to maintain the same control law.

It is also unclear that the thermistors will operate correctly or even be safe at the higher voltage rating - you need to find THAT out from their datasheets. 

For example, if the higher power dissipation causes increased self-heating, what will that do to the control law? 

If the thermistors check out as safe at 230V, you can try the circuit, but expect some fiddling to get it working as intended.