WPF- Animate from one style to another

Question

Hi guys

Just wondering if anyone know how to animate from one style to another i.e. going from NormalStyle to ActiveStyle when the user focuses on the textbox

<Style x:key="NormalStyle" TargetType="{x:Type TextBox}">
    <Setter Property="BorderBrush" Value="Gray" />
    <Setter Property="BorderThickness" Value="2" />
</Style>

<Style x:key="ActiveStyle" TargetType="{x:Type TextBox}" BasedOn="{StaticResource NormalStyle}">
    <Setter Property="BorderBrush" Value="Green" /> 
    <Setter Property="BorderThickness" Value="4" />
</Style>

Cheers Anthony