Damn, this topic is full of bullsh-t.
First, there is no way that pensions of non-executive employees are in any legal risk on this. As was accurately stated, the pension as funded today is legally isolated. Also once a person takes their lump sum the money is theirs, it is no longer attached to 3m in any way. The lawyers would have to penetrate 3M and get a suit against each pensioner individually to even attempt to get that money back. They would have to prove a lot on an individual employee basis to even have a chance at getting that money.
That said, future contributions from 3m to the pension fund are at risk. But the money already there is pretty damn well safe.
I think these letters are lies, I don't see any competent lawyer sending one. If they are being sent, I would expect media attention as it would be an unprecedented legal threat to rank and file workers on very shaky ground.
If someone wants to prove me wrong, post a copy of this letter with the employee's name and address redacted.