Thank you for your question today, I look forward to assisting you. I bring nearly 20 years of legal experience in various disciplines.
Unfortunately, without a contract of employment specifically guaranteeing you the new position and also guaranteeing that you could only loose the new position "for cause" there is nothing to be done here, just on the facts you've given.
In your state, just like every other state except one, without a contract of employment expressly stating that you can only be terminated for cause, your employment is "at will."
That means that your employment can legally be terminated at any point, and that the terms of your employment can be altered at any time, with or without your consent.
Verbal promises of continued employment are "at will" which really means that they bear no legal power beyond the company's actual choice to adhere to it. It's an "at will" offer, that can be changed at any point.
So, unless you have some evidence that the only reason you are being reduced in position is your race, religion, gender, age, disability or FMLA use, there is simply nothing that can be done here.
I won't say you haven't not been treated unfairly, but that doesn't equate to a legal issue in most situations. Employment law is actually very limited in what is illegal.