It founds 5 text.
...
app.findGrepPreferences.findWhat = "\\t";
app.changeGrepPreferences.changeTo = "xxxx";
app.changeGrepPreferences.appliedParagraphStyle = myPar9;
...
var founds = app.activeDocument.changeGrep();
alert(founds.length);
it alerts 5. If app.changeGrepPreferences.changeTo is "" (empty string) the myPar9 isn't applied to rest of the text. If it's "xxxx" or something else, the following text successfully replaced with myPar9's style.