Is it possible to upgrade one phone on a plan to unlimited texting without a fee?

My sister and mother share a plan, and last night my sister ordered me the Blitz, adding 250 texts to it. I have to pay my part by myself, so until I know approx. how much money I will be making from my new job, I didn't want to pay too much. Is it possible to change our plan, upgrading my texts up to unlimited before the 2 year contract for my phone is up? And if so, is there a fee? Thanks !