David, do you mean .xlsx?
If so, the xlsx extension seems to be what Microsoft gives it for their new
files that are native to MS Excel 2007.
Changing the extension just in a folder by renaming the file is likely to
cause the file not to behave properly. However, if you must have the xls
file name, you are really saying that you want Excel to save the file as the
older type of Excel. Which is easy to do.
on a file by file basis just do File, Save As, and choose file type to by
Excel 97-2003. You can also make this the default way you want your files
saved (MS calls it "compatability mode") by clicking the Office Button
(circle in the top left corner of Excel) and click Excel Options at the
bottom of the openned window. Then click the SAVE label on the left and the
1st option on the page to the right is "Save files in this format:". Select
from the drop down box "Excel 97-2003 workbook (*.xls).
Hope I am answering what you are asking.