You can do it by taking the controller apart and doing some cutting and solderign, but, could very well end up with it being more of a pain than it used to be.
All you used to have to do is sever an then swap the 2 outside leads of the POT, and that's it, you're done. The Stick is now inverted. It's just a voltage divider and doing that just swaps the + and - to it is all.
The issue you will most likely run into though, is that the XB1 controllers are built, then calibrated. So if the POT is in there at just a little bit of an angle, thus it's center value is not exactly center, they offset that by calibrating it after it's made. It's different for every single controller, as well as every single axis in the controller. So if your controller has a POT that's not sitting close enough to dead center, when you swap the outside leads around you'll make that offset flip with it as well and could end up with a controller that 'drifts' in one direction or the other. That too can be corrected, but that's where it gets into the PITA part of it.
It can always be put right back how it was, or even a DPDT switch can be installed so you can swap back and forth between the Normal and Inverted layouts, provided you didn't have to do any adjustments for drifting, as then it gets to be even more of an issue and you really need to get a PIC and DAC in there to be able to have that kind of option sometimes.
You can try swapping them and see how it acts, not going to hurt a thing, unless you just tear the controller up or short something in the process, but it's not any kind of ban worthy thing at all, and should be an option added to their menu from day 1 there.