Reducing or removing Retirees Medical Benefit

My Company currently provides some retirees with medical cover and pays the full cost for the retiree and their family. Over the years the cost has become more and more expensive and I am seeking ways to reduce or withdraw the benefit. There is very little in writing and the benefit has been in place for decades, although renewed annually. Are there legal implications if I try to remove or limit the benefits e.g. is there an implied contract even though the policy is renewable annually?

Does anyone have any bright ideas - I can only think, that like the PM, I would ask those impacted how they would reduce the cost.