I ordered a 6S via BestBuy and it didn't force me to chose a new plan. My confirmation email also states "Keep existing plan."
I'm still not activating the SIM it will ship with though. I will remove it and re-sell the phone. I bought another 6S directly from Verizon at full price to keep and use.
I think you screwed yourself over. When adding a 6S to the cart, the popup has a blue box at the bottom that shows:
There's a change from Verizon Wireless for unlimited data plans.
Verizon Wireless has ended this plan option, so when you upgrade you'll be switched to their most common 2GB plan by default.
No, this warning can be safely ignored as long as you don't activate the SIM they ship you. They are multiple forum threads confirming this at SlickDeals.
*Also, if by chance you get any kind of email or phone call saying there's an issue with your order, do not call them. If you give them any account information over the phone, they can process the plan change. Just cancel the order via the website.