I was given a Samsung S4 for Christmas by my daughter who purchased it for one of her lines. She gave it to me and I activated it on my line to replace a 4G Samsung Charge. I gave her the Charge which she activated on her line. It took a little while on the phone with Verizon because the SIMs had to be changed also (the phones have different size SIMs); if the SIMs were the same size we could have done the whole thing in a few minutes ourselves.
When a phone is purchased at a discounted price, Verizon considers the contractual obligation to remain with the line that buys the phone, not the line that uses it.