I have 2 wireless accounts. So my other one is the one i bought the S5 with & this one i am using i haven't upgraded to is for almost 2 years. The rewards points i got them via other having all 2 accounts together & promotions.
Althought i am still debating if i should not use the points so i can save more & more until next year. I might do that.
Before you do anything , back up the device on iTunes. This way you can restore it on the iPhone 6 . Then You need to factory reset that phone to the point that there is no data or personal info in it. This can be done via iTunes or the phone settins. Which ever one u perfer. Once that is done Take the sim card out of the device
& you are set to sell it.