It is my understanding that if you pay full retail price for the phone, you can keep your current plan. It only changes if you accept their subsidized price.
I'm in the same boat. My monthly bill is $69.99 with unlimited data (they matched the Sprint plan through their retention department when I told them I was switching). If I take their subsidized phone, my bill goes up to $100/month and a lose unlimited data. It's a lose-lose. If I pay full price and keep unlimited, I make up the difference in a year.