I did it this morning with my wife's phone, and got her an S6 Edge+. (In-store) I had pre-ordered, and it became a pain in the @55. Spend an hour on the phone with a CSR, and wound up just trading in for the $95, and had her annotate my account to credit the remaining $105 to the phone upon receipt. I'll have to call in again, but at least they are making arrangements, and working with me.
It seems if you do it on the site- it will give you the fair trade price, and then give you the right price during check-out. Wish they would just create a promotion code that made it easier to do.