I will share my experience, but your mileage may vary! Before I ever powered on the XS Max, I removed the AT&T sim and then put the sim from my existing phone in. I then powered it up and it took me through an activation and I thought on crap it's going to bill me the $30 activation fee. However, as I understand it and since I've never been billed the $30 the assumption is that existing sim had already been activated so it didn't charge me the fee. I was told that once I started the phone the old sim would no longer work and I'd be forced to use the new sim. That was not he case for me.
Again I am just sharing what I did and I can't promise you one way or the other that it will work. But if it doesn't then just call into AT&T and ask them kindly to waive the fee. I see a lot of forum users have been successful in doing so. Just remember if you get a Rep that refused, just wait a while and call back again. One guy called 3 times, but did get it waived. I guess persistence paid off. 🙂
I initially purchase a sim free version and I did the same thing with it, but at the time that I got the 2nd XS Max they had no sim free version in stock at the store and I got the AT&T version.