Does anyone have links or resources that address the impact to a lump sum when a corporate pension changing from the GATT rate to the 10yr Corporate rate?

If someone was expecing a $225,000 lump sum from their pension using the GATT rate, what would the impact be if the company changed to the 10yr corporate rate? Obviously, it will be a lower lump sum, but I have a client who will be retiree in the middle of a “Phase in” of the corporate rate so we are trying to determine the impact.

I appreciate input, web links or other resources which address this issue.