Involuntarily? I don't think so. 2007 Excel files have the default
extension of .xlsx unless they contain macros. Your 2003 .xls file
should still be available on disk, although it won't have the changes
you made to it in 2007.
As you probably have found out by now you can save files used in Excel
2007 in the 97-2003 format. I have both 2003 and 2007 on my computer. If
I open an .xls file 2007 will save it as an .xls file.