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In your implementations, add the bean name.

public class OldFeatureService implements FeatureService{
    @Service("oldFeatureService")
    public String testMe() {
        return "Hello from OldFeatureService";
    }
}

public class NewFeatureService implements FeatureService{
    @Service("newFeatureService")
    public String testMe() {
        return "Hello from NewFeatureService";
    }
}

By default, springboot uses the bean with @Qualifier("OldFeatureService"). When the flag is enabled, it looks for the alterbean=newFeatureService

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