I'm sure that'll be the case in two years' time. I've heard of people who took advantage of the double data promotion (4GB for the price of 2GB) not being able to upgrade and keep the 4GB for the price of 2GB plan.
I can't speak for the double data promo but I was able to keep the $20 data plan when upgrading through Comcast. Best Buy didn't let me keep the plan though
Sent from my Verizon Samsung Galaxy Note 3