1.cs

public interface ChangePasswordResult
{ }

public class PasswordChanged : ChangePasswordResult
{ }

public class ChangeNotAuthorized : ChangePasswordResult
{ }

public static class ChangePasswordResultEx
{
    public static TResult Match<TResult>(this ChangePasswordResult result, Func<TResult> success, Func<TResult> notAuthorized)
    {
        if (result is PasswordChanged)
            return success();
        return notAuthorized();
    }
}

2.cs

public abstract class ChangePasswordResult
{
    public static readonly ChangePasswordResult PasswordChanged = new PasswordChangedImpl();
    public static readonly ChangePasswordResult ChangeNotAuthorized = new ChangeNotAuthorizedImpl();

    public abstract TResult Match<TResult>(Func<TResult> success, Func<TResult> notAuthorized);

    class PasswordChangedImpl : ChangePasswordResult
    {
        public override TResult Match<TResult>(Func<TResult> success, Func<TResult> notAuthorized)
        {
            return success();
        }
    }

    class ChangeNotAuthorizedImpl : ChangePasswordResult
    {
        public override TResult Match<TResult>(Func<TResult> success, Func<TResult> notAuthorized)
        {
            return notAuthorized();
        }
    }
}