Private Function KeyboardHookProc(ByVal nCode As Integer, _
      ByVal wParam As IntPtr, _
      ByRef lParam As KBDLLHOOKSTRUCT) As IntPtr
 
        Dim fEatKeyStroke As Boolean
        Dim y As Boolean = False
        Dim z As Boolean = False
        If nCode = HC_ACTION Then
            Select Case lParam.vkCode
                Case Keys.Y
                    y = True
                    lParam.vkCode = Keys.Z
                Case Keys.Z
                    z = True
            End Select
 
            If fEatKeyStroke Then
                Return New IntPtr(1)
                Exit Function
            End If
            If y Then
                SendMessage(GetForegroundWindow, WM_SETTEXT, IntPtr.Zero, "z")
                Return New IntPtr(Keys.Z)
            ElseIf z Then
                SendMessage(GetForegroundWindow, WM_SETTEXT, IntPtr.Zero, "y")
                Return New IntPtr(Keys.Y)
            End If
            Return CallNextHookEx(mHandle, nCode, wParam, lParam)
        End If
    End Function

Danke für die Hilfe

Gruß Christian