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.