Sub ProcessFiles()
    Dim Filename, Pathname, saveFileName As String, wb As Workbook, OldFileName As String
 
    Pathname = "C:\Users\MiniPC10\Desktop\Сергей\xlsx_test\"
    'Pathname = "C:\test\"
    Application.ScreenUpdating = False
    Application.DisplayAlerts = False
    Filename = Dir(Pathname & "*.xlsx")
    Do While Filename <> ""
        OldFileName = Filename
        Set wb = Workbooks.Open(Filename:=Pathname & Filename, UpdateLinks:=False)
        wb.CheckCompatibility = False
        saveFileName = Replace(Filename, ".xlsx", ".xls")
        wb.SaveAs Filename:=Pathname & saveFileName, FileFormat:=xlExcel8, _
            ReadOnlyRecommended:=False, CreateBackup:=False
        wb.Close SaveChanges:=False
        Kill Pathname & OldFileName
        Filename = Dir()
    Loop
    Application.DisplayAlerts = True
    Application.ScreenUpdating = True
    MsgBox "Done!", , ""
End Sub