If it were another phone, then you'd have no problem. Since the BlackBerry has a BlackBerry plan, you might have trouble. I actually logged in this morning and noticed that AT&T knew that I switched my SIM card out of the iPhone. I regularly switch phones, but they never noticed. Today, when I logged in, it said my phone was a Moto Q 9h. My data plan was the iPhone plan. I called and they said there is no difference between the iPhone plan and MediaNet. This is great, so I can swap the SIM card out whenever I want to whatever phone I want. Again, this will vary with the BlackBerry. I also have a BlackBerry and was forced to set up a different line of service for that phone.
Good luck. As the previous poster mentioned, give them a call and hopefully you'll get someone who is on the ball. The gentleman I spoke with this morning was very helpful and knowledgeable.