I want to buy Verizon cell off of co-worker and add it to my Alltel plan. Can I do that?

My co-worker's bf recently added her to his Verizon plan. A month later, they broke up. Initially, she was going to put the cell in her own name. Instead, her parents added her to their Alltel plan. So, she is going to give cell back. The bf wants $350 to cover the cost of turning cell off. I was wondering, since I am due for an upgrade in 2 weeks through Alltel and I was wanting that brand of cell that is advertised at about $150, can I just buy the phone from him for $150 (my friend can pay him the remainder of the early termination fee) and have it activated through my carrier? If I can, will I still get to keep my upgrade and current #? And they have 4gb. My plan is offers 5gb. Does that really matter? Also, if I am able to "take over" her cell, will my carrier charge me a any "special" fees?