May an Employer reneig on a signed, executed and paid severance agreement?

My employer (based in Michigan) made the decision to close the plant where I work. they put together a spread sheet with proposed severance agreement and stay on bonus dollars. My last day of work was 12/5/2009. I agreed to continue to assist the Company after I left and was allowed to keep my cell phone and had access to the company e-mail. When I received my Severance Agreement, I was provided with additional money from the company. The VP of HR and the COO signed the Agreement. I reviewed the Agreement, signed it after a few weeks and returned it to the Company. I received my check on January 8, 2010. I received a call from the VP of HR today advising that I was paid too much money. I believed that I had a well reviewed, signed contract. I had to find new employment and start my life over. Can the Company now change the terms of the Agreement?