One of the few things I don't like about yahoo is the lack of commish controls. You can't go back. The only thing you can do is like the previous poster said, keep track of it yourself. Add the week 1 scores yourself in your head to the final standings. If the seedings change because of that, then you'll have to ignore the yahoo playoff schedule, and use your own. This can be done, you just wouldn't be able to use statracker and see the H2H matchups each week, cuz the matchups they'd have wouldn't be right. And then you may have problems with pickups, and also it could affect someones yahoo Iprofile as well.
I think you'd be better off just throwing out week 1. It sucks if you won that week, but it's the easiest way to solve the problem.