cant not change history on a shared workbook
 

I have an excel 2003 file, that is a shared workbook, the file is huge, and I
know if I uncheck and recheck the shared option, it will clear my history and
decrease the file size. but I would like to change the amount of history it
is keeping. (defaulting 30 days) When I go Tools \ share work\ advanced
tab. only the keep history for 30 days is grayed out, and all the other
options are availble, I can not figure out why, can anyone please help me