I retired from a company on 08/01/2010 under a Defined Contribution Plan. The company modified the plan to include a Defined Benefit on 05/01/2011. The plan included a retroactive component to go back and calculate and pay the benefit for every past year of employment based on a formula. There are only 5 of us who retired on the DC plan prior to 05/01/2011 and we are being excluded from the retroactive benefit. I understand that since I am a retired employee, I would not get the benefit going forward; however, it seems wrong to exclude me from the retroactive benefit?
My guess is that this is a small company and the participation is carved out to cover only the highly-compensated employees. They have to hire an actuary to install a DB plan and he planned it, probably very carefully to exclude the 5 persons not in pay status. So I am not optimistic for you.
If you want to pursue it, you can file a complaint with the US Employee Benefits Administration. To learn how to file a complaint go to the below website. Its 4th paragraph covers that.