Hope you're getting some sleep now. Others have already said it, but you won't have a problem cancelling a within 24 hours booking on UA, even on an award ticket. UA is very generous about this, and it's one thing that keeps me with the carrier.
One thing though... if you do get off the standby list on your original flight, please please cancel the UA ticket immediately. It's tempting to 'deal with it later', but with New York such a mess right now, somebody else is going to need that seat. You cancelling could be the difference between them getting home or spending another night on the floor at EWR.
If you do take the UA flight, don't forget that your original airline is obligated to give you a full cash refund for the flight they cancelled. Although call centers are busy, I would call and ask for that fairly quickly too -- that will cancel that ticket, and may prevent somebody else from sleeping on the floor at JFK.