Call during Israeli business hours so you can speak to them as they have more power and usually more flexibility to make these changes. If they say no, hang up and call again. Eventually, you might be able to convince someone to do it. Some agents will do it the first time around and some will tell you it is no longer possible. Don't ask if they can do it - tell them what you are doing and usually they will go along with it.
Good luck - worst case you get to enjoy some quality El Al hold music...
