Many of my family members have done this recently. No kosher food in the airport in ATH, as in most European airlines.
Elal, Arkia, and Israir all fly ATH-TLV, so you should be able to find a decent timed connection.
Not sure why you’re mentioning Emirates. If you’re planning on canceling the United flight and purchase new tickets, you can connect in many other European cities and take Elal from there.
With ticket prices to Europe these days, I don’t see any benefit in exchanging the United flight to ATH. You’ll probably get more if you cancel and look for a new pair of flights.
You can PM me if you want assistance with this.