How the hell did you do that? I have been wanting to upgrade but was told I would loose my unlimited plan. And I have been a customer for over a decade!
on June 28th, they rolled out new "tiered" data plans. If you resigned your two year contract before the 28th, you could keep your existing contract (by selecing no change) and get a discounted phone.
moving forward, you can keep your existing unlimited data plan by not signing a new contract - once your contract is run out, you just continue paying month to month. if you want to upgrade, you'll have to buy a phone on your own and just activate it (the phone discounts only come when you sign a new 2 year contract).
the new tiered data plans are not that bad, since the data allowance can be shared across multiple lines. i think they range from 2gb to 8gb. depends on what you mainly use it for, standard use and web browsing wont put you close to maxing out, but if you do a lot of streaming, downloading, etc - you might.
for example, i'd max out with a 4g LTE even at 8gb, since I use my phone as a mobile hotspot a lot.