Update to 4.2 and your Wii will brick with 003 error.
100% Guaranteed. Without fail. Brick. You will have to buy a new Wii.
Don't do it.
If you do it despite this post, don't come back here crying ok.
Besides 4.2 offers no extras over 4.1, go to 4.1 if you are in need of an update. Not 4.2 though.