Default format changed in Excel 2007
 

I have a fairly large file that was originally converted from Excel 2003.
Recently it changed its default format from General to Date. It is about
2.5MB in size and has 30 or so tabs, so I really can't go and change cells
back to general by selecting and reformatting them.
When I open another file or a new file or even an earlier version of this
file, the default remains at General. When I do a Save As the default format
remains at Date.
Does anyone know how to change the default format for a file BACK to General?