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I don't think you should wait until the device gets no longer supported, as the trade-in value will considerably dip after that. Currently you are getting the device free or if there is some hidden cost, an almost free upgrade.

As per me, you should do the trade unless you want to wait for iPhone 12 or something. Apart from the amazing upgrades mention above, remember the new iPhone SE will have better resale value than iPhone 7 in another two years or so. You will kind of have a double profited deal!

In the USA, if you upgrade at a T-mobile store, there is a $20 upgrade fee and you pay sales tax on the $399 (depending on state.)
 
In the USA, if you upgrade at a T-mobile store, there is a $20 upgrade fee and you pay sales tax on the $399 (depending on state.)
I went ahead and ordered the phone online, so there is no upgrade fee. I only paid tax (around $20) -- and shipping, unfortunately. Still, around $25 for a new SE is a good deal!

My concern here is that I have it nowhere in writing that I will actually receive the $399 credit. I only have an oral confirmation by the representative online.
 
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