How do I change the control type from a button to a togglebutton in Expression Blend?

Question

You don't have to start from scratch especially with it being a Button -> ToggleButton where they are almost identical in base functionality.

So by just doing a find and replace Button to ToggleButton you'd be almost done. As you mentioned ToggleButton does not like:

<Trigger Property="IsDefaulted" Value="True"/>

so you need to switch that to

<Trigger Property="Button.IsDefaulted" Value="True"/>

however if you look at that xaml, it's an empty Trigger definition which is pointless. So in your Style you could just get rid of the following bit:

<ControlTemplate.Triggers>
  <Trigger Property="IsFocused" Value="True"/>
  <Trigger Property="IsDefaulted" Value="True"/>
  <Trigger Property="IsMouseOver" Value="True"/>
  <Trigger Property="IsPressed" Value="True"/>
  <Trigger Property="IsEnabled" Value="False"/>
</ControlTemplate.Triggers>

Am not going to start explaining what makes this an empty Trigger, As I mentioned you should pick up a book on "Basic WPF and Blend" to get to grip with how to do stuff in blend and what blend actually does for you in the background.

As for TypeDefinition:

It just means what's the type of control, that Style / ControlTemplate is intended for.

In the following lines:

<Style x:Key="HolonBaseButton" TargetType="{x:Type Button}">

<ControlTemplate TargetType="{x:Type Button}">

the TargetType="{x:Type Button}" says that the Style in first line and ControlTemplate in second are intended to be used for a Button so if you try to use it for a ToggleButton, you'd get an error. It's pretty verbose, so to switch it you just switch Button with ToggleBUtton which the find and replace would do fine.

So your converted Style:

<Style x:Key="HolonBaseButton" TargetType="{x:Type ToggleButton}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="383*"/>
                        <ColumnDefinition Width="5"/>
                    </Grid.ColumnDefinitions>
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Normal"/>
                            <VisualState x:Name="MouseOver">
                                <Storyboard>
                                    <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Stroke).(SolidColorBrush.Color)" Storyboard.TargetName="Background">
                                        <EasingColorKeyFrame KeyTime="0" Value="{StaticResource MouseOver}"/>
                                    </ColorAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Pressed"/>
                            <VisualState x:Name="Disabled"/>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Path x:Name="Background" Data="M0.5,0.5 L383.5,0.5 399.5,16.499999 399.5,59.5 0.5,59.5 z" Stretch="Fill" Grid.ColumnSpan="1">
                        <Path.Fill>
                            <SolidColorBrush Color="{DynamicResource HolonBaseBackground}"/>
                        </Path.Fill>
                        <Path.Stroke>
                            <SolidColorBrush Color="{DynamicResource HolonBaseStroke}"/>
                        </Path.Stroke>
                    </Path>
                    <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Visibility="Hidden" Margin="182.188,22.02,165.188,22.02"/>
                    <ToggleButton x:Name="btnMark" Content="ToggleButton" HorizontalAlignment="Right" Margin="0" Style="{DynamicResource MarkToggleButton}" VerticalAlignment="Top" Grid.Column="0"/>
                    <TextBlock x:Name="TimeCreated" HorizontalAlignment="Right" Height="Auto" Margin="0,3.7,20.794,0" TextWrapping="Wrap" Text="9 mins ago" VerticalAlignment="Top" Width="Auto" FontFamily="{DynamicResource SystemText}" FontSize="10" TextAlignment="Right">
                        <TextBlock.Foreground>
                            <SolidColorBrush Color="{DynamicResource DisabledControlsText}"/>
                        </TextBlock.Foreground>
                    </TextBlock>
                    <Rectangle x:Name="Selector" Grid.Column="1" HorizontalAlignment="Stretch" Height="Auto" Margin="0" VerticalAlignment="Stretch" Stroke="{x:Null}" Visibility="Hidden">
                        <Rectangle.Fill>
                            <SolidColorBrush Color="{DynamicResource Selector}"/>
                        </Rectangle.Fill>
                    </Rectangle>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>