No, I'm sorry, the upgrade credit can only be used towards the purchase of an upgrade. You can visit www.sprint.com/etf to learn more about early termination fees as they are on a sliding scale - the lesser number of months on the contract, the lesser the termination fee.
Does anyone know if it is possible to use an upgrade credit on a line to cancel another line? I have a family shared plan with 4 lines, I need to cancel 2 of the lines. One of the lines I need to cancel is still under contract, but the other is not.