How to add functionality to an existing Interface Class and Implementations

[semomaniz]

Please help me understand the concept of how I can add functionality to an existing Interface class and its implementations with out breaking any previously released codes.

Here is the Interface Class

Public Interface IShipmentProvider
    ReadOnly Property ProviderName As String
    ReadOnly Property ShipmentItems As ICollection(Of Package)
    Function GetRates() As ICollection(Of Rate)
    Function VerifyAddress(ByVal address As Address) As ICollection(Of Address)
End Interface

Here are the implementations

Public Class FedExShipmentProvider
    Implements IShipmentProvider
  
    Public Function GetRates() As ICollection(Of Rate) Implements IShipmentProvider.GetRates
        Throw New NotImplementedException
    End Function

    Public ReadOnly Property ProviderName As String Implements IShipmentProvider.ProviderName
        Get
            Return "FedEx"
        End Get
    End Property

    Public ReadOnly Property ShipmentItems As ICollection(Of Package) Implements IShipmentProvider.ShipmentItems
        Get
            Return _lstofItems
        End Get
    End Property

    Public Function VerifyAddress(address As Address) As ICollection(Of Address) Implements IShipmentProvider.VerifyAddress
        Throw New NotImplementedException
    End Function
End Class


Public Class USPSShipmentProvider
    Implements IShipmentProvider

    Public Function GetRates() As ICollection(Of Rate) Implements IShipmentProvider.GetRates
        Throw New NotImplementedException
    End Function

    Public ReadOnly Property ProviderName As String Implements IShipmentProvider.ProviderName
        Get
            Return "USPS"
        End Get
    End Property

    Public ReadOnly Property ShipmentItems As ICollection(Of Package) Implements IShipmentProvider.ShipmentItems
        Get
            Return _lstofItems
        End Get
    End Property

    Public Function VerifyAddress(address As Address) As ICollection(Of Address) Implements IShipmentProvider.VerifyAddress
        Throw New NotImplementedException
    End Function
End Class

Public Class UPSShipmentProvider
    Implements IShipmentProvider

    Public Function GetRates() As ICollection(Of Rate) Implements IShipmentProvider.GetRates
        Throw New NotImplementedException
    End Function

    Public ReadOnly Property ProviderName As String Implements IShipmentProvider.ProviderName
        Get
            Return "UPS"
        End Get
    End Property

    Public ReadOnly Property ShipmentItems As ICollection(Of Package) Implements IShipmentProvider.ShipmentItems
        Get
            Return _lstofItems
        End Get
    End Property

    Public Function VerifyAddress(address As Address) As ICollection(Of Address) Implements IShipmentProvider.VerifyAddress
        Throw New NotImplementedException
    End Function

End Class

Here I have provided three implementations of the IShipmentProvid er but there may be more.

Now let just say I am supposed to add additional functionality of generating shipping label. How do I add the function of creating label to the interface class and implementations without breaking any other implementations . If I add the function directly to the interface then I will run into issue of breaking all the existing implementations unless I update all of them. Is there a way we can add the functionality with out breaking or updating existing implementations .

Please direct me to the right directions.

Thank you in advance.

[semomaniz]

Any help would be really appreciated.