iPhone 11 pro max

You can speak with the store, by calling them, but don't be upset or surprised if they tell you they cannot reserve the model you want the day you want it. You have to understand that millions upon millions of people have already pre-ordered the new phones. Apple will satisfy those orders first, but will have some very limited stock in-stores that are sold on a first come first served basis. I can't say they won't do what you want, but it would not be normal for them to put one away for you, regardless of whether you are returning the Xs you already purchased.

If you pre-order, you will need to pay online for the order. And rereading, you can't pre-order in store. If you want to buy in store, you take your chances and for the first few weeks, supplies will be VERY low for walk-in customers.

You can't reserve a device at an Apple Store.

They are sold on a first come basis.

You have 14 days to return your phone. But that does not mean they may have what you want available when you want it.

You have 14 days to get a refund on the device.

That does not mean the store will have any newer devices for sale, however.