You mean you want to keep the basename and just change the extension? That can be achieved like this:
newname = fso.BuildPath(wb.Path, fso.GetBaseName(wb.Name) & ".xls")
wb.SaveAs newname, -4143
If you want to delete the old file afterwards, you can do it like this:
f.Delete True
I'd do this after closing the workbook, just to be on the safe side.
BTW, I already told you that
wb.Close SaveChanges=True
doesn't do what you seem to think it does. Use just
wb.Close True
without the parameter name.