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Change-Over Valves are 'either or' - that is they pass air from either of two circuits to whatever is downstream. The valve prevents back-feeding into the 'other' supply circuit.
Think of it as a shuttle-valve.
Are you, perhaps, confusing a Quick-Release Valve? This is normally in a circuit where 'quick-release' is desirable (such as a spring-brake parking supply - or rear airbrakes). When pressure is applied a diaphragm is held over the exhaust port so that the air supply can be passed downstream to the actuator - when the input pressure disappears the spring lifts the diaphragm off the exhaust port allowing the residual air to escape directly rather than having to return 'upstream' to the application valve exhaust port.
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