Well, I needed to have my questions answered, because they frame the answers to your questions.
Without a contract of employment, you are an "at will" employee. That means that you can legally be terminated without cause or warning. To overcome that rule, you have to be able to claim some sort of illegal discrimination for the termination to actually be unlawful.
So, when you say that you have been unlawfully terminated, I have to explore what you could mean by that. If you were not an employee with a contract stating that you could only be terminated for cause AND you can't allege discrimination based on race, religion, gender, age, disability or FMLA use, then you were not actually wrongfully terminated (at least by legal standards).
To address your specific questions:
Can the new management over-turn a written review for the whole 2012 in just 30 days?
Yes, they can change it. Personnel files are the property of the employer. Because the information contained therein is just for their company use, it is not considered defamation of character for it to contain inaccuracies. Essentially, they are allowed to lie to themselves. If they broadcast the lies to third parties, such as prospective employers that you try to get a job with, they then engage in defamation of character.
Did HR played its role correctly? Should they have disclosed to me that they are not neutral players in this case?
HR works for the company, so their role is to protect the company from suit. Many think that HR works for the employees, but that's just not true. Often HR's goals will coincide with the employee's goals and that's a good situation, but that is not any sort o legal requirement. HR is not neutral, but they have no legal obligation to say they are not neutral. Instead, they have a legal obligation to refrain from saying that they are neutral. They just can't tell you "I work for you, so everything you say to me is confidential."
Given that I have sufficient proofs to prove my above statements is there a law against such practices?
I addressed this, I believe, in my other two responses. Minnesota law only grants you the right to review personnel files, not to adjust them. The employer has a property right in them and until they broadcast false information to third parties, there is no legal action to take. HR did it's job and broke no laws, at least based on the facts that you gave. While I don't appreciate the way HR often behaves, they have a pro-employer position, always, in every employment situation.
Just on those facts, I do not see a wrongful termination claim. I really do wish that I could say differently here, but I don't deal in false hope. I'm simply respecting you enough to be honest with you about the law in this situation.