This topic has been beat to death, search all over the forums for your answers. But as a summary: No one keeps unlimited data when they upgrade. If you want to sign a new two year contract then you agree to lose your unlimited data. You could instead just process the insurance claim as they suggested and keep it. You could also purchase a device at full retail or buy a used one from a website such as swappa.com. This has been the case for quite some time now and sorry you just learned about it, but that's how it is.
T-Mobile is NOT unlimited. They throttle you after a certain amount of usage when connected to their towers and you have a very LOW allowance and very HIGH overages when not connected to their towers. Read more into it.