Option Explicit

Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim oCC As ContentControl
    If ContentControl.ShowingPlaceholderText = False Then
        If ContentControl.Title = "Country" Then    'the title of the list box control
            Select Case Trim(ContentControl.Range.Text)
                Case "United States"
                    For Each oCC In ActiveDocument.ContentControls
                        If oCC.Title = "Short Name" Then    'the title of the fields used for the short version
                            oCC.Range.Text = "US"
                        End If
                    Next oCC
                Case "Mexico"
                    For Each oCC In ActiveDocument.ContentControls
                        If oCC.Title = "Short Name" Then
                            oCC.Range.Text = "MEX"
                        End If
                    Next oCC
                    'etc
            End Select
        End If
    End If
    Set oCC = Nothing
End Sub