Not sure why you think this. But Apple Store or Apple.com has never in its history have resold a returned items back to a new customer. Once a customer returns the phone, Apple marks those as open box and then be sent off to Cupertino to be looked at and then turned into warranty replacement units.
Also Apple store near me, if you purchase a brand new iPhone and it's still in its wrapper and you return it, they are forced to open it and verify and mark it as open box item and not for resale. I've been told that they have been getting hit with customers returning them and repackaging them as if they were't open and only changing the device or putting fake iPhones in and repackaging them. Each Apple store may be different.
You can definitely try your luck at the Apple Store and see if you were able to get a newer build but that all depends on how long they had in stock. For example, Apple Store near me have 65 units of iPhone XS Max 256 GB Space Gray sim - free model in stock on the shelf. Now it's been like this for a week or so and Apple Store won't be getting any new shipment for that model until the stock runs low. Who knows when that will be when they have 65 of them. So even if January comes and still have that amount, and someone buys one then, then likely they will get one from September build.
Good luck