How to make the transition from "traditional" to reactive MVVM

class ReactiveModel
{
    public string ValueA {get;set;}
    private readonly Subject<string> valueB = new Subject<string>();
    public IObservable<string> ValueB
    {
        get
        {
            return valueB;
        }
    }

    public void UpdateB(string newValue)
    {
        valueB.OnNext(newValue);
    }
}

class ReactiveViewModel
{
    private readonly ReactiveModel model;
    private readonly ReactiveProperty<string> valueA;
    private readonly ReactiveProperty<string> valueB;

    public ReactiveViewModel(ReactiveModel model)
    {
        this.model = model;

        valueA = new ReactiveProperty<string>(model.ValueA);
        valueA.Subscribe(x => model.ValueA = x);

        valueB = model.ValueB.ToReactiveProperty();
        valueB.Subscribe(model.UpdateB);
    }

    public IObservable<string> ValueA
    {
        get
        {
            return valueA;
        }
    }

    public ReactiveProperty<string> ValueB
    {
        get 
        {
            return valueB;
        }
    }
}

<TextBox Text="{Binding ValueA.Value, UpdateSourceTrigger=PropertyChanged}"/>