Interfaces for an upgrade service to handle changes between versions on an application. If required, IUpgradeService should be implemented in a PCL with the CurrentAppVersion left as abstract to implement on a per device basis.

IUpgrade.cs

/// <summary>
///		Defines an interface which upgrades
///		the application from a version which is great
///		or equal to <see cref="IUpgrade.FromVersion"/>
///		and before <see cref="IUpgrade.ToVersion"/>.
/// </summary>
public interface IUpgrade {
	Version FromVersion { get; }

	Version ToVersion { get; }

	/// <summary>
	///		Attempts to upgrade the the application.
	/// </summary>
	/// <returns>
	///		True if the upgrade was successful. ie.
	///		false will be returned when an upgrade
	///		was attempted but no data was present.
	/// </returns>
	System.Threading.Tasks.Task<bool> DoUpgradeAsync();
}

IUpgradeService.cs

public interface IUpgradeService {
	/// <summary>
	///		Ascertains whether the application
	///		requires an upgrade by comparing
	///		the current <see cref="CurrentAppVersion"/>
	///		with the stored version.
	/// 
	///		An upgrade is only required when an <see cref="IUpgrade"/>
	///		has been registered which can upgrade the previous version
	///		(or greater) to the current version.
	/// </summary>
	bool RequiresUpgrade { get; }

	/// <summary>
	///		The previously upgraded (or stored) application
	///		version.
	/// 
	///		This will be null if it's the first time the user
	///		has opened the app.
	/// </summary>
	Version PreviousAppVersion { get; }

	/// <summary>
	///		Returns the current version of the application.
	/// </summary>
	Version CurrentAppVersion { get; }

	/// <summary>
	///		Adds an upgrade to the upgrade service.
	/// </summary>
	/// <typeparam name="TUpgrade">
	///		The type of upgrade to add.
	/// </typeparam>
	/// <exception cref="InvalidOperationException">
	///		Thrown if the same type is attempted to
	///		be registered twice.
	/// </exception>
	void RegisterUpgrade<TUpgrade>() where TUpgrade : IUpgrade;

	/// <summary>
	///		Attempts to upgrade the application.
	/// 
	///		Silently fails if <see cref="RequiresUpgrade"/>
	///		is false.
	/// </summary>
	/// <exception cref="InvalidOperationException">
	///		Thrown when the one of the registered upgrade's
	///		<see cref="IUpgrade.ToVersion"/>
	///		is greater than the application's current version.
	/// </exception>
	Task DoUpgradeAsync();
}