Actually, given how companies manage the WARN act, I'd wager they actually do. Often they keep an employee on the books for the 90 days as an employee, just with no responsibilities. I would presume changing the responsibilities would be no different then being re-assigned to a different team.

Yeah, but they have lost all leverage. If employee refuses to assist, its not like the company can fire them or withdraw the severance. No sticks left and only carrots. For which a very big carrot would be appropriate.