When midnight of Preorder day rolled around, all iUP customers had only the option of reserving a phone from their local store's preorder inventory (which is separate from the normal web order queue). Obviously inventory is limited on launch day and those quere quickly all reserved. So yes, the option was there at midnight for T-Mobile customers to upgrade-- though very few were able to actually do so. To compensate, Apple had to set up a hotline to call and work with a CSR directly to find a device to be able to upgrade. I was on the phone for hours. (But I DID end up with the device I wanted on launch day. If I had wanted Jet black, that wouldn't have been possible, though, and the iPhone X may be a similar scenario.)
The difference this year so far is that customers on other carriers won't be limited to that option. We'll have to wait until after the event tomorrow to hopefully get some more official information on how T-Mobile iUP preorders will be handled.