Well...there's no reason, because you won't gain anything with 4.2. But you can do that if you want...easiest method:
-Update HBC/DVDX/BootMii with the latest
HackMii-Installer
-Install/update all IOS/Channels with
DOP-IOS MOD, and install the 4.2 Menu from there.
I recommend to have BootMii as bootv2 installed, because there's a Brick risk...