Invalid Cast Exception when Working with Inherited Classes

[Ideas Man]

I know the title isn't exactly perfect, but the problem is hard to put it into words.

Basically what I have is two classes, they are almost perfectly the same, however the derrived class contains some extra elements such as in this example.

'Base Class
Public Class CarBase
 
    Protected _Colour As String
    Protected _Model As String
    Protected _Make As String
 
    Public Property Colour() As String
        Get
            Return _Colour
        End Get
        Set(ByVal Value As String)
            _Colour = Value
            Call ValueChanged()
        End Set
    End Property
    '
    'Property Repeated for the Model and the Make
 
    Protected Overridable Sub ValueChanged()
        'To be utilised in derrived classes
    End Sub
 
End Class
 
'Derrived Class
Public Class Car
    Inherits CarBase
 
    Private _ChangesMade As Boolean = False
 
    Protected Overrides Sub ValueChanged()
        _ChangesMade = True
    End Sub
 
    Public ReadOnly Property ChangesMade() As Boolean
        Get
            Return _ChangesMade
        End Get
    End Property
 
End Class

Now, the problem is that i have other functions and stuff to manipulate the classes, they all work from the base class (CarBase) because that stores just the data items, nothing to manipulate them etc. The derrived class really only adds the ChangesMade and some other properties that don't really affect the stored data.
I need both classes to be able to access the functions, but I don't really want to have to rewrite them to accomodate the two types of classes, some of which only return values so I can't overload them.

When I try to assign a CarBase to a variable of type Car, I get an Invalid Cast Exception. Is there anyway I can allow it to do this type of action w/o raising an exception? The IDE doesn't detect an error and neither does the compiler, so there must not be anything wrong with the syntax.