Sub Demo()
Dim Rng As Range, i As Long
With ActiveDocument
  For i = .FormFields.Count To 1 Step -1
    With .FormFields(i)
      If .Type = wdFieldFormCheckBox Then
        Set Rng = .Range
        .Delete
        With Rng
          .InlineShapes.AddOLEControl ClassType:="Forms.CheckBox.1"
          .MoveEnd wdWord, 1
          .InlineShapes(1).Width = .InlineShapes(1).Height
        End With
      End If
    End With
  Next
End With
End Sub