Sell it and hope Apple makes one for the new models. If you try to mod it and destroy it, then you lose the money you could of made selling it and have to spend the money for the updated case on top of that.
Have you used an Apple Smart Battery Case in the past? If so, the one for the XS/XS Max is different. I absolutely love the Smart Battery Case for the iPhone 7, but the one I had for my XS Max made the phone heavy and the buttons weren't as responsive because the material is a lot more rigid and not as soft as the previous generation's, so the buttons were stiff. The material of the silicone for the iPhone 7 is perfect. It bugs me when a case I loved for a previous generation is changed for current generations. For example, I love the Apple Leather Cases for the iPhone 5s because of how thin it was and how it covered the bottom.
With that being said, if button response isn't that big of a deal to you, then I absolutely recommend the Smart Battery Case; everything is covered nicely, the silicone doesn't attract as much lint/dust as the Apple Silicone Case, the phone fits snug, and having that extra battery is fantastic.