Wirelessly posted (BlackBerry8830/4.2.2 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/105)
You can go standby on the immediately prior flight for free, IIRC. Other same-day changes cost money. Even with the free snacks I'd suggest bringing food with you if you plan on eating on the 4.5 hour flight. Terra chips are only so filling. Sadly you are flying in to JFK so you won't get to take advantage of the great food options they have in the terminal for buying food for outbound flights.
Also, United is UA, not UN.