Because DL allows you a free stopover on an award ticket, unless you already have one in your itinerary, you should be able to change just the SFO-JFK portion of your return to a date in the future. Keep checking dates in 2015 to find one with availability for just that leg at your award level (low, it sounds like). Then, as others suggested, in 2015 you can pair it with a one-way award ticket, and you'll have an RT from NY to SFO for the $150 change fee you'll pay now ( unless you're Plat or Diamond), plus the one-way mileage price.