app.findGrepPreferences = app.changeGrepPreferences = NothingEnum.nothing; 
app.findGrepPreferences.findWhat = '<fnote>\\X+?</fnote>';  
var the_result = app.documents.firstItem().findGrep();  
for (var i = 0;  i < the_result.length; i++) { 
 
 
//--->the_result[i] = the_result[i].replace('<fnote>','<enote>'); 
//--->the_result[i] = the_result[i].replace('</fnote>','</enote>'); 
 
myCopy = the_result[i].duplicate(LocationOptions.AFTER,app.selection[0].parentStory.insertionPoints[-1]) ;  
 
 } 

myCopy.contents = myCopy.contents.replace('fnote>','enote>','gi');