Without seeing your setup it’s hard to tell you if what you were told is a load of carp or not.
If you move to a setup like this generally you remove the current irrigation controller entirely and replace it with this system. The wires for your zones that went to the current controller would instead go to the OpenSprinkler controller. This gives you control over each zone/solenoid individually. That said you could have some whacky controller but I doubt it. The real things to worry about is does your system use solenoids to control the water flow, and are these solenoids powered by 24vac. If yes to both of these question then you are good to go.