To save up to 60% on First Class upgrades, download the Seatfrog app, add your Grand Central Standard Class ticket to the app and check for available upgrades. 

Then, between 24 hours and 30 minutes before your train. place your bid (the maximum amount you're willing to pay for the upgrade). If you are successful, you will be notified by Seatfrog and you can hop into First Class - simply show your original Standard Class ticket and the upgrade barcode on the Seatfrog app to the conductor.