My introduction to taxation professor changed the grading

My introduction to taxation professor changed the grading scale without notifying us. The syllabus has not changed at all during or towards the end of the semester. We never knew that she changed the scale until our grades for the course were published. The syllabus states that an 87% to 89% is a B+ and that percentages that are .5% will be rounded up. My final percentage earned in the class was 86.74%, which qualifies for that round up. The professor said that yes, she did round up to an 87%, but she, “believes that an 80% to 89% is a B,”(Direct Quote) and gave me a B for my course grade. I attended every single one of her lectures and always participated. I would have remembered if she said there were changes made to the grading scale during our short, 5 week course. She did not mention changes were being made at all. I realize that there’s not much a difference between a B and B+, but it affects one’s GPA negatively if their average is high. What can I do? Do I talk to the department head or go to the dean with this issue? Is it still possible to get a B+? Also, is this an ethical thing for a professor to do? This has never happened in my entire educational career, and I have been a student for a long time. Please help. I am unsure what to do.