"One way to transfer a benefit liability is to transfer the benefit to the participant by paying out the benefit in a lump sum. The other way is to purchase an annuity from an insurance company.... [This de-risking] approach has a much greater impact on plan participants, as it may change who will pay their benefit and requires them to decide whether or not they want to receive a lump sum distribution. It is this ... approach which raises the most issues under ERISA."