Yes, you will lose your unlimited.
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.
- ESN (electronic serial number) change away from/back to Smartphone with unlimited data (lost or stolen device, warranty replacement, or loaner device) within 14 days of the effective date of the change
- ESN change away from/back to MB (Mobile Broadband) device with grandfathered plan (lost or stolen device, warranty replacement, or loaner device) such as $35/3GB and $20/1GB within 14 days of the effective date of the change
- Pricing plan or feature change within 14 days of the effective date of the previous change
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.