A few notes from that website if folks didn't see.
Terms This offer is only for customers who received a text message notifying them. During congestion, the small fraction of customers using >50GB/month may notice reduced speeds until next bill cycle due to data prioritization. Video typically streams on smartphone/tablet at DVD quality (480p) unless Binge On is disabled. Capable device required. High-speed tethering included in the amount of your previous high-speed data allotment, then reduced to 2G speeds. U.S. roaming and on-network data allotments differ: includes 200MB roaming. Unlimited high-speed data US only. Video streams at up to 1.5Mbps if Binge On is enabled. Optimization may affect speed of video downloads; does not apply to video uploads. For best performance, leave any video streaming applications at their default automatic resolution setting. Service may be terminated or restricted for excessive roaming. Smartphone and tablet usage is prioritized over Mobile Hotspot Service (tethering) usage, which may result in higher speeds for data used on smartphone. Coverage not available in some areas. Network Management: Service may be slowed, suspended, terminated, or restricted for misuse, abnormal use, interference with our network or ability to provide quality service to other users, or significant roaming. See T-Mobile.com/OpenInternet for data management details.
Data Stash With unlimited high-speed data you no longer need Data Stash since you’ll have unlimited data at no additional cost. Beginning on 3/16, when the upgrade happens, your existing Data Stash will no longer be available.
Tethering You’ll continue to receive high-speed tethering data at your previous limited high-speed data allotment. For example, if your line has 6GB of high-speed data, you’ll receive 6GB of high-speed tethering data after the upgrade, followed by unlimited tethering data at up to 2G speeds.