Notice: This is not risky if you have Bootmii installed as a boot2 replacement. Well, not very risky.
Notice 2: DO NOT ASK ME FOR ANY OF THIS STUFF. Its not illegal but it can promote piracy. So don't ask. Google is your friend.
Notice 3: This takes some technical know how.
Notice 4: Preloader .3 hasn't been released except for an autoupdate yet, wait for the true online release if you want it.
Ok, anyway, you can upgrade to 4.2U and still use Homebrew.
What you need:
System Menu 4.1
A trucha signed IOS249 (cIOS38 rev14)
A Wad Manager (1.4 or 1.5)
NUS Downloader (Found on Wiibrew)
Homebrew already installed
Preloader .30
Bootmii as a boot2 replacement (not required but recommended highly)
Method:
1). Make cIOS38 rev14 installer an active application, as well as the Wad Manager (it cannot be a WAD Manager channel please notice that) and preloader.
2). Run the cIOS38 rev14 installer, either have the WAD for it on the SD card or use the network install (the latter is recommended, use the NUS Downloader to get it otherwise).
3). Run the NUS Downloader. Check the 'Pack the wad' option and for title do the following:
Wii System Menu 4.2: 0000000100000002 v481
Wii Shop Channel: 0001000248414241 v17
IOS38: 0000000100000026 v3867
IOS56: 0000000100000038 v5405
IOS57: 0000000100000039 v5661
IOS70: 0000000100000046 v6687
4). Make a folder in the root of the SD card called 'Wad'. Paste every wad produced inside (ignore the files without an extension, just grab the wad files).
5). Edit the hacks.ini file to have the following code for the preloader file and put it in the root of the SD card:
6). Run the WAD Manager from the Homebrew channel. Choose to use the power of IOS249 and choose the front SD slot and install everything but the Wii system menu.
7). Go to the bootmii screen after this and make a nand dump (highly recommended, not required).
8). After making the nand dump, go back to the Wad manager and install the system menu wad.
9). Do NOT EXIT TO THE SYSTEM MENU. In the loader, go to the preloader application. Install it and then it should take you to the preloader screen. Go to the hacks options and make sure to enable the code.
10). Now you can go to the system menu. Until an update patch comes out, you MUST load every time from Gecko OS for any new game or your Wii might become LU64+ (not really but really, it will become boot2v4 but if you already had a trucha signing bug your fine).
11). Enjoy.
Please note this has no warranty whatsoever and there is absolutely no point in going to 4.2 at the moment. This is for NTSC-U Wii's and do this at your own risk. its fairly safe but remember that this has no warranty whatsoever. Post a visitor message or contact me on my Youtube account (my home page) if you have questions or troubleshooting.
You should still be able to downgrade after this but be careful seriously. Just installing the system menu for 4.1 should be fine but make sure you install preloader.

I have tested and confirmed this for an upgrade from 4.1U to 4.2U. Enjoy.
Here's the link to the video:
http://www.youtube.com/watch?v=kz9XZ9axWLM&fmt=18
Notice: This is an upgrade guide from 4.1 to 4.2. You need Homebrew already installed.
Notice 2: You should have no noticeable loss of functionality with it, it should work fine.
Notice 3:This will make it so you can keep HBC 1.03/1.04 with your newest Wii System Menu so you can have full functionality with both preloader and bootmii.
Notice 4: I recommend having nand dump before doing this and bootmii installed as a boot2 replacement.
What You Need:
The following Wii Applications/Installers on Your SD Card:
WAD Manager 1.4
Preloader
HackMii .5
HackMii .2
The Following PC Applications:
NUS Downloader
How to Set Up Your SD Card:
1). First, in the root of the SD card make a folder called 'wad' without the quotations.
2). Go to the directory you download the NUS Downlaoder, unzip it, go inside and run the batch file for your proper region. Remember, J is for a Japanese Wii, U is for North American, and E is for European. If you have questions, ask here.
3). Go into each folder that is produced by the NUS Downloader and copy the produced '.wad' file to the 'wad' folder that you just made on your SD card.
4). Make sure you have an 'apps' folder in the root of your SD card. If not, make one. Now, make a folder called 'Wii Wad Manager' in your 'apps' folder and copy the .dol file into that folder and rename it boot.dol.
5). Next, make a folder in 'apps' called Preloader. Now, extract the the files from the rar of the preloader download (if you don't have a rar unzipper, use 7-zip)). Now, copy the hacks.ini to the root of the SD card and the .dol file to the preloader folder you just created. Rename the .dol file to boot.dol.
6). Now make a new folder in 'apps' called 'Hackmii .2' and copy in the 'boot.elf' from the download inside. Make another folder in 'apps' called 'Hackmii
.5' and copy in the 'boot.elf' from the right download inside.
7). Now, edit the hacks.ini file and delete the contents. Paste inside the following instead for your proper region:
NTSC-U (North American)
PAL (European)
NTSC-J (Japan)
Upgrading
1). Now, put your SD card on your Wii and go into Homebrew. Run the Wii Wad Manager and install every wad (choose IOS36 for the install, front SD slot for the install location). Make sure you install every wad file before you reset your Wii.
2). Now, make sure you exit by pressing Home and go to the Homebrew channel menu. Do not leave that menu. If you reset the system menu, your Homebrew channel will be deleted. Now, start the preloader application.
3). Ignore the warnings and have it install preloader on your Wii. Now, reload to the preloader menu (hold the reset button for a while) and go to the System Menu hacks option. Make sure you choose the 'Disable HAXX, DVDX..' and 'Block Disc Updates' options. Save the settings and go back to the main preloader menu.
4). Now, go settings and make sure the System Menu IOS is IOS60, and choose whatever autoboot you want (regular is System Menu, which is recommended for both).
5). Go to the Homebrew channel and run Hackmii .5 and install everything possible. Now, return to the Homebrew channel and run Hackmii .2 and install the Homebrew channel (since Hackmii .5 deletes the previous Homebrew channel and 1.05 has issues with compatibility but has better features).
6). Congratulations and enjoy, your Wii just got awesomer.