As a non-lawyer, my thoughts on this are certainly little more than random musings. But FWIW, it seems to me that the fact the new fund, whatever it may be, underperformed the old fund, may be immaterial. By that I mean that the plan investment committee which selected the investments is under no obligation to continue to offer that same fund. So if your "old" fund earned 25%, or 200%, or whatever, in one month since the transfer, that's meaningless IMHO. If the selection of the new fund was undertaken in a prudent manner, as suggested earlier, then I'm not sure you really have a lot on your side. The fact that you threw away the notice probably doesn't help your cause.
Just out of curiosity, if you HAD opened the notice, and seen the available fund lineup from which to choose, what fund would you have chosen? What would this fund have earned in the 1 month period, compared to the default fund into which your account was placed? If you DO have a solid case (and again, as a non-lawyer based only upon some sketchy facts and discussion, I feel like that may be doubtful) I think this may be a more appropriate measure than comparing it to the old fund which would have been unavailable to you in any case.