We're considering offering an early retirement program whereby employees' age + service would be bridged for purposes of qualifying them for retiree medical (currently Rule of 70). Does management's decision to attribute additional age + service to an employee technically require a plan amendment in order to be recognized under the plan or can its decision to deem early retirees an additional number of years of age + service to get them to the vesting threshold be viewed as an administrative decision?

I think you need an amendment.