Can an at will employee grieve with EEOC an unfair demotion?

Just found out I am being demoted because another employee is being promoted to my same position. I have had no negative performance ratings or disciplinary actions. I have been employed there 13 years, the last 8 in the position I am now in. Is this actionable? I am an at-will employee.

My boss retired last month. When he got promoted 8 years ago to Assistant General Manager, I too was promoted to an Executive Assistant II. Our admin team consists of an Office Assistant III, Executive Assistant 1 and myself. After he retired, our senior director was promoted to his position. While he was a director--the exec asst. 1 reported to him. He now wants to promote his assistant to an ea2, but HR told him that he could not have two ea2 positions. His remedy was to demote me back to ea1 and of course I will have to train his assistant to take over my duties. I feel that the atmosphere would be intolerable and that it is unreasonable to expect me to take a demotion when I have had no negative reports or evaluations. I am 56 years old and only planned on working another 6 or 7 years. I want to maintain my current position and salary ceiling. Is this something I can fight?