I had to go through the path of going to the page where you enroll in the Apple Upgrade Program. It will refer you to the Apple Store app on your phone to "prepare for preorder". I was wondering if at the end I could apply a trade-in value for my XS Max so I reached out to customer service. Here is a transcript of the conversation:
View attachment 966169
So based on that, you should be able to apply a trade-in value. I haven't completed the "prepare for pre-order" because I am still waffling between 12 Pro and 12, but am hoping there is a trade in option to apply at the end of the process. If not, worse case scenario I sell it on Swappa.