First time getting a new phone with a new carrier. questions about iphone and preorders and deals

been with at&t for 18 years. woke up at 3am every year on preorder day and got the new iphone, either with the ancient 2 year contract method or more recently with next. just switched to tmobile in may, and i remember seeing a deal for "free*" iphone 7 with trade in.

*not really free, i know

i have a fully paid off iphone 7 plus.

assuming they run a similar deal this year, am i still able to preorder in the a.m. on september 12th (rumored date) and trade in mine for bill credits ?