You only need to replace the triacs with NPN transistors on the stations you are going to use. The remaining triacs can remain in the circuit. However, you need to clearly label those unmodified stations so they are not used for anything else. With 12V DC applied to the board, the triacs will turn on, but not off.
Don’t forget that you also need a clamping diode across the solenoid to dissipate the spike when the valve is turning off. The cathode of the diode connects to +12V. The anode of the diode connects to the collector of the transistor. 1N4001 rectifier diode is OK here. Depending on the size of the solenoid, a must smaller diode may be OK too. However, 1N4001 or similar diodes are very common. Both the transistor and the diode can be found in the meager parts bin of your local Radio Shack.
Here is a picture of a similar connection using a relay. The concept is the same.