Just did this Monday. Was very easy. I pre-ordered my 11 Pro MG 256GB online. Received it on launch day. Kept it a week and decided I wanted the Max instead. Verified my local Apple store had the 11 Pro Max in MG 256GB in stock. Originally purchased using the Citizens One Apple Payments. All they had to do was return the original 11 Pro, rebuy the 11 Pro Max and change the payment papers. Took all of 15 minutes. Waited longer to get checked in.
What sold me was looking at my 11 Pro and my wife's 11 Pro Max side by side, text on the Max just seemed to be more crisp. Loved the extra screen space for watching videos, and she on average got about 2 more hours of battery life then I was. I came from the XS last year. The extra size and the little bit of added weight take a day or so to get use to, but now on my 3rd day its like holding my XS. Phone feels great with the Apple Leather forest green case on it.