When blizz releases the 2.4.3 patch do
NOT upgrade your client!
Make sure to start wow using wow.exe and not the default launcher.exe!or it will automaticly update your client...We will support it after some time, but for those who upgrade before we tell you to will be unable to connect.
It will take some time to sniff the new packets and implement them.
Good luck & have fun!
What to do to downgrade to 2.4.21) Go to your World of Warcraft\Data folder ( c:\program files\world of warcraft\Data\ )
2) Delete the file 'patch.mpq'
3) Go up one directory ( usually c:\program files\world of warcraft\ )
4) Start repair.exe and select 'Check files only'
5) It will take a little while to finally find patch.mpq missing. When it does it will tell you it has to fix it.
6) Once it is done, you will be back at 2.0.0.
7) Update all required patches to 2.4.2 (
most likely they are still in you wow folder, if not download them again)

Edit your realmlist.wtf and startup using wow.exe ( c:\program files\world of warcraft\wow.exe )
Update files in your local installation folder:
Remember to update your shorcuts to be set to wow.exe instead of launcher.exe ^^
I love how 1/4 didn't actually read the post. If you've already upgraded then follow these instructions as they were given above. However, if you have different operating systems, you may need to do something different. I'm not sure. But if you are using windows, simply follow the instructions and you'll be set.
Upgrading a server to a new patch isn't as easy as upgrading a client to a new patch. I'm sure EVENTUALLY the server will upgrade to the new patch, but probably not for awhile.
If you use the wowrage launcher, that is based off the server. It will not help you at all, if you try to connect with your current version of 2.4.3.
If you downgrade, you will not be able to connect to retail servers. The only reason you want to downgrade is because you're getting the "Invalid Game Version" message. However, if you want to be able to play retail and wowrage, I suggest make two different wow folders. One with version 2.4.3 and one with version 2.4.2. Thats what I did when I played retail and it worked fines for me.
Again, lets re-cap this. Upgrading to 2.4.3 is bad if you want to continue to play on wowrage.
Upgrading to 2.4.3 is needed if you want to play on retail.
Downgrading is needed(thats if you've already upgraded) if you want to play on wowrage.
You MUST downgrade then patch back up again if you wish to play on wowrage(again, only need to downgrade if you have already upgraded).
Hopefully that solves any questions about the process.
Have any questions about it, feel free to send a pm or come ask me in ventrilo(wee little 8man) 69.223.86.111 port 3784 pw epic1