I'm sorry to hear that you had to have your device replaced. It sounds like everything is taken care of since you are receiving a replacement by mail. I am happy to advise that you are able to keep your unlimited data plan, provided that certainly guidelines are met.
In your case, Sarahkaylpn, the first bullet point describes your situation. So long as you activated you replacement device within 14 days of our original Thunderbolt's deactivation then you can switch back to the unlimited data package. If you have any additional questions or concerns then feel free to reply to this posting for additional assistance.