samsung blackjack buy

If I buy a Samsung BlackJack II outside eBay, what happens with my current AT & T plan?
I currently have Nokia N75 with AT & T. I do not qualify for an upgrade yet, so instead of buying the Samsung Blackjack II for the price of AT & T I bought it on eBay for cheaper and just put my old SIM card inside. But my question is: What will happen with the minutes / text / data plans that I have now? I am part of a family plan. We have unlimited data for $ 15 per month and unlimited messaging for families 30/month. Does the data plan will go up only for my phone line or what? Thanks for your help ..
Their system of family type and family unlimited messaging will not change, but the agreement with the Blackjack II will personal data PDA unlimited plan for $ 30/month instead of the regular Unlimited Data $ 15/month. You should call or go to a warehouse to store information about your new phone AT & T systems and obtain data in terms of the right or the Internet can not function on your phone. Also, if one of the lines of others in your family has an unlimited data plan $ 15/month, you can save $ 5 per month for the change to the Family unlimited data package is only $ 10/line until the family messaging unlimited.
San Marcos from Samsung Blackjack