Is there a way to update an old RSS feed item and get it as "unread" in a feed reader?

My feed has been created from a liferay portlet by liferay itself, and it shows just 1 item because I can update only that one, I cannot create new items/portlet.

if I edit my old feed, in google reader or Firefox live bookmarks it's not shown as unread, and even if I refresh and I see the new content, still the date is the old one.

I'd need a way to inform the readers that the old feed has been updated... is that possible using one single item?