Can you change script function?

Hi, everyone

I got this script:

find-change EN to EM in the cells

but only in active document

//=======================================================================================

var doc = app.activeDocument,  
  _tables = doc.stories.everyItem().tables.everyItem().getElements();

app.findGrepPreferences = app.changeGrepPreferences = null; 

for(var i=0;i<_tables.length;i++)
{
        app.findGrepPreferences.findWhat = "~=";
        app.changeGrepPreferences.changeTo="~_";
        _tables[i].changeGrep();
    }

app.findGrepPreferences = app.changeGrepPreferences = null;

//=======================================================================================

I want it function in all open documents

thanks

Teetan