Retirement Pension

I recently retired from a major corporation. I received my pension options and chose the one that best suited my needs. I would be receiving a flat rate per month for the remainder of my life. I have received three monthly payments to date at the rate that had been proposed to me.
I received a letter last Friday from the Service Center that administers the pension plan for this Corporation and the letter stated they had made a mistake in my calculations and they had been overpaying me $400.00 per month. They told me they would continue to pay me for three more months at the higher rate, but then I would have to pay them back $2400.00 that they have overpaid them.
My questions are, "Can they go back and change the amount I am being paid after they have already began payment of my pension?"
Can I be forced to pay back this amount that they are now saying they have over paid me?
Do I have a chance in fighting this?

Technically yes you do need to repay if you agree it is an error; you can however argue hardship in having to pay all or any substantial portion--saying that it was their mistake after all, and you spent it w/o realizing it was in error, and paying back now would be difficult...they often will not sue or take any further court action in these cases but will try to work something out. They may even settle on a partial amount.

World Law's lawyers could also probably help resolve this if need be. You may email them on the home page.