A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
If it's a .xls to begin with, that's what Save As defaults to, but... Look at the very top of the list of file formats - above the spot where you actually click to open the list. IMO a poor UI design flaw, but you'll see Excel Workbook (.xlsx) listed above everything else - but it seems to be lost among the file names.
There is always a warning that something may be lost if you attempt to save in something other than .xlsx or .xlsm format, but it doesn't necessarily mean that anything actually will be lost. For example, .xls only supports 256 columns by 65,536 rows, but when you open one in Excel 2007/2008/20010/2011 the sheets are provided with the full complement of 16,384 columns by 1,048,576 rows. If you continue to save in the .xls format those additional columns & rows cannot be retained so any content in that portion of a sheet would be lost.
Regards,
Bob J.