My response would be that the question is becoming increasingly a moot point. Most corporations have already closed their defined benefit plans. They've already moved to a system oriented to individual pension accounts.
No new DB plans are being opened by the corporate sector, so you're really talking about almost a wind-up situation. As for the idea that you can retroactively change these priorities, I'm not a lawyer, but I think there's a huge legal argument that says you cannot. I don't think it's a big deal to do it prospectively, because that's not where pensions are going.