Thank you for your reply.
It is illegal to retroactively reduce an employee's pay. So, to the extent your employer is trying to pay you less than your original rate for work you already performed, that would be illegal. However, since employment is "at will" absent an agreement to the contrary, the terms of employment--including pay--can be changed at any time for virtually any reason upon notice to the employee.
This means regardless of whether the basis for your pay deduction and demotion was "fair," it would not violate any law. It would only be a violation if you could prove that your employer had an unlawful discriminatory motive, such as your race, religion, age or gender for the adverse action (i.e. they believe you are too old to do the job and want to replace you with someone younger, so they lowered your pay to get you to quit). You would have to prove the illegal motive, though--the burden falls on your shoulders.
In short, it is illegal for your employer to cut your pay retroactively. You are entitled to your original rate of pay for all work you performed up to the point that your employer informed you of the paycut. Your employer is free to cut your pay once they notify you of the paycut, though. This is because employment is "at will" and can be changed at any time for any non-discriminatory reason. It doesn't matter whether you sign a document acknowledging the paycut or not--your employer can reduce your pay without your consent and your only options are to negotiate something better or leave for alternative employment where you get paid what you deserve.
One final point, which is that a paycut this drastic would in most cases qualify as good cause to quit such that you would be eligible for unemployment benefits (normally quitting will result in disqualification for unemployment). So, you may wish to consider quitting, collecting unemployment and beginning to look for another job.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.