Upgrading with 2 year contract and keeping unlimited data

I did this with the original Turbo, but I have an upgrade available on my unlimited data line and my sister has 2GB of data on our family plan. I plan on transferring my upgrade to her line, upgrading the phone and activating it. Then just slip my nano sim and... profit. Anyone successfully upgrade their phone and keep their unlimited data? Does the best buy method still work as well?

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