workbooks.open打不开97-2003文件

VBA门外汉，请求帮助！先谢谢啦！

目前的代码如下。
推测问题点：需要打开的EXCEL文件是97-2003版、而且是受保护的视图。
最终目标：打开这些文件、修改为可EDIT、并另存为*.xlsx文件。
注：若将".xls"改为".xlsx")，可以成功打开2007版EXCEL文件。

Sub OPENFILES()
Application.ScreenUpdating = False
Dim FileName As String, WB As Workbook, SHT As Worksheet
Dim FN As String

FileName = Dir(ThisWorkbook.Path & "\" & "*.xls")
Do While FileName <> ""
    FN = ThisWorkbook.Path & "\" & FileName
    Set WB = Workbooks.Open(FN)
       
    FileName = Dir()
Loop
Application.ScreenUpdating = True
MsgBox "目标文件全部打开！", vbInformation, "MACRO提示"

End Sub