Some of my colleagues rely on syllabus amendments. At some schools, you must have a class vote to make modifications to your syllabus. Other institutions require department head or dean approval.
I don't make changes to my syllabi. I ensure that they are air-tight by running them past colleagues, the head of the department, and faculty outside of my department.
I've learned from my syllabus mistakes from the past, and I haven't had any syllabus-related problems in the last year. With some new preps for me on the horizon, we'll see if I can maintain that streak.