I have been working 40 hours per week, but I would like to change it to 35 hours. Do I need to get a new job offer letter? Same position and employer. Nothing changed but hours. Boss and I already agreed with this.
You do need a new offer letter, or something else in writing confirming the change.
Suppose your manager quits or moves to a different department. A new manager might require you to work 40 hours a week if that is what your offer says.