I second mrburns42’s reply. Yes, you only need to replace the triac for the station that you are modifying.
For connecting to relays, you can either interface with a relay directly, and make sure you have a flyback diode installed as suggested by mrburns42. Or you can use a relay board (which typically includes a flyback diode per channel). In fact, if you use a relay board that is of ‘active high’ type (i.e. a logic high activates the relay), you can just solder a wire from the corresponding shift register output to the relay channel pin, and you don’t even need to replace triac.
As a side note, on OpenSprinkler (not the Pi) v2.0 (assembled SMT version), I’ve designed PCB holes to fit the flyback diodes. These are hidden under the SMT triac. So if someone wants to interface with DC devices, they can remove the triac, and then solder a transistor together with flyback diode.