I upgraded from an iPhone4 to a 6Plus in the Apple Store several weeks ago (My two year contract expired almost two years ago - I waited so long to upgrade because I didn't want to lose my unlimited data). I was not charged an activation fee in the store, nor was there one on the bill that I received last week (which did reflect my new contract and plan, so if I was going to be charged, it would have been on there).
We are doing the exact same thing this week (going from iphone 4 to 6plus) ...I hope there are no additional fees. I had 2 live chats with verizon. Each time, the rep told me I could transfer an upgrade from a non smart phone on my plan to the phone we wanted to upgrade so I could keep the unlimited plan. I went to order the phone, and they wont let us keep it. Spoke to 2 supervisors telling them I have the live chat discussions saved...they still wont do it. I really hate Verizon!!