I just did this today -- 4.1U to 4.2U.
I did it just because it appeared to be safe, and I figure it's always good to be on the latest version, as long as you wait until the time to upgrade is right. Which is now, since I upgraded with no problems.
Here is what I did:
* Install CIOS Rev 15
* Install newest beta4 / .61 of HackMii installer (all 3 items, bootmii, DVDx, and Homebrew channel 1.06)
It's important to install the new 1.06 HBC since the new system menu will delete the old one that you currently are using.
* Install the "Safe 4.2" installer (takes about 30 mins)
I don't recall at the moment where exactly which thread I got this from. It's called "V4.2_Safe_Firmware_Updater.rar"
It's very nice -- you simply start it, and it does the rest. Let me just paste in some text from the readme:
Code:
which allows you to safely upgrade your Wii console to v4.2 firmware. It will detect your console region before installation, and download only the REQUIRED files to successfully upgrade your Wii console to v4.2. It will not overwrite your boot2 or stub any of your IOSes. It will not trucha patch any downloaded IOSes either. You must have an internet connection on your console to use this application. Source included. Enjoy!
* Use DOP-IOS MOD (the new one) and trucha bug IOS 36, 53, and 70
To clarify, depending on your needs, you might need to restore the trucha bug to a few IOSs. Using the newer DOP-IOS MOD (not "DOP-IOS" as it has older versions of IOS) you can easily re-download these IOSs and have it apply the hash sig bug.
I just did this to two Wiis today, one softmodded and one hard modded, and am not having any problems. And NSMB even works on both (with IOS 53 trucha'ed, it runs fine off the disc channel on the hard modded one -- it needed to be since I applied the "hack" to the DOL file, to get my backup to work).
If you can't search and find that 4.2 safe updater, let me know, and I'll help you locate it.
*** I forgot to mention, be sure to have 250-300 blocks free when you run the Safe 4.2 Updater. In that thread I was speaking of, that was mentioned!!