If they do not have bootmii boot2 (which they won't due to Korean Wiis having a fixed boot1) or preloader installed, and they cannot boot to anything other than #003 error, then there is no homebrew solution.
The best way is to prevent getting the #003 error, by either not updating to 4.2x, or by waiting for cIOSCORP v3.5 to be released, installing cIOSCORP, and only then (after installing cIOSCORP v3.5), manually install a System Menu 4.2 wad of the region they changed to.
Although... just off the top of my head...
Maybe region-changed Korean Wiis could try recovery mode... If they can boot into it...
Maybe with a modchip, which is capable of autobooting discs from the recovery menu, along with trucha bugged IOS installed... and have cIOS249 installed...
If these conditions are met... then maybe they can boot a wad manager autoboot disc, and install IOS60 v6174 (to remove the IOS60 stub), then install System Menu 4.1 of the region they changed to...
Don't know if this will work though...
Do you have a Korean Wii?