Adjusting the process literally is part of Scrum. At the retrospective, if something isn't working, you call it out and change it. That can be something as big as completely changing the methodology. I've never worked on a XP team but my understanding is formally there is no retrospective process - I would definitely want to put one in place if I did work with it.
Any development process that doesn't allow for changing itself is shortsighted and ultimately self-defeating.
Any development process that doesn't allow for changing itself is shortsighted and ultimately self-defeating.