Solenoid valve on 24V powered with 12V?
If they work reliably then you will extend the life of the solenoid valve forever. Since the power dissipated in the coil is given by \$ P = \frac {V^2}R \$ you will reduce the power to 25% when you reduce the voltage.
Possible problems might be that the valve will not open fully or fail to initially pick up. Usually you will find that there is hysteresis between pickup and release and I would have expected that you might need about 16 V to energise the solenoid and that it might not release until you lowered the voltage to about 8 V or so.

Figure 1. Solenoid valve cross-section. 1. Valve Body 2. Inlet Port 3. Outlet Port 4. Coil / Solenoid 5. Coil Windings 6. Lead Wires 7. Plunger 8. Spring 9. Orifice. Image source: Wermac.
If you have a variable bench power supply it would be worth checking the actual pick-up voltage to give you some confidence of your operating margin.