It will be offered when you check in so do it then if you want it, otherwise it might be full by the time you get to the airport with others upgrading. I did it once and I think it's totally worth it. Bags are free and if you already paid for reserved seats in coach the fee will be refunded. It is very nice and roomy up there at the front of the plane. I think there are better snacks and drinks, can't recall for sure.