MainWindow.xaml

<mah:MetroWindow x:Class="TransitioningContentControlTest001.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TransitioningContentControlTest001"
        xmlns:vm="clr-namespace:TransitioningContentControlTest001.ViewModels"
        xmlns:uc="clr-namespace:TransitioningContentControlTest001.UserControls"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <vm:MainViewModel />
    </Window.DataContext>
    <Window.Resources>
        <DataTemplate DataType="{x:Type vm:Content1ViewModel}">
            <uc:Content1UserControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:Content2ViewModel}">
            <uc:Content2UserControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:Content3ViewModel}">
            <uc:Content2UserControl />
        </DataTemplate>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>
        <!--<ContentControl Content="{Binding Content}" />--> <!--For comparison.-->
        <mah:TransitioningContentControl Content="{Binding Content}" />
        <Button Grid.Row="1" Content="Content1" Command="{Binding LoadContent1Command}" />
        <Button Grid.Row="2" Content="Content2" Command="{Binding LoadContent2Command}" />
        <Button Grid.Row="3" Content="Content3" Command="{Binding LoadContent3Command}" />
    </Grid>
</mah:MetroWindow>