I pre-ordered my 14 Pro and got it on launch day (16th) and did a trade in for my 13 Pro. Like many others, I got an email on the pre order stating I could either use the trade in kit or take it to a store. I received the trade in kit yesterday but thought I would try going to the Apple Store to see if I could just trade it in there.
They tried to do the trade in, but since I had already opted in for the trade in kit, it wouldn't let them complete the transaction. I wasn't given any option about in store or kit when I completed the transaction on pre order day and I had to have the phone delivered to my residence as I was not going to be able to do an in store pickup.
So, I think the net of it is this - If I had opted for in store pickup, I think they could have taken the trade in when I picked up the device. Since I did a delivery to my home address, they cannot override the mail in trade.
I'm late to this thread and didn't think about cancelling the mail in trade.....so, my 13 Pro is on the way to the trade in partner (Jeffersonville IN) so we'll see how that goes. Worst case, if they try to rip me off, I'll just reject the offer and have my 13 Pro sent back to me.....