I change my cd keys all the time manually and it takes me less than 5 seconds.
Just to let you know, 5x5 and 4x4 keys both work. There are no problems, not one, with using them interchangeably in regedit. You don't have to go through any of that trouble.
The thing about launching with steam is that steam automatically over-writes the registry with the steam key. So this causes all kinds of head aches no matter what. I just don't play with steam usually anyways.
For me, all I do is hit start --> run and type in regedit. Luckily, I don't even have to do that the second time since run saves the most recent thing typed already in it.
regedit launches and you go to My computer\HKEY_CURRENT_USER\Software\MountAndBladeWarbandKeys and double click on serial_key and paste in the CD key you want that you save in a text file on your desktop for ease of use. You don't have to find that file path a second time since regedit saves the most recent filepath in it every time you launch it.
And bam! you are done! That is it. All I do is open a text file on my computer, copy the cd key, click start, click run, click ok for regedit, double click on serial key, Hit paste. Less than 5 seconds. You don't even have to close warband to do it, since it is just a registry check upon joining the servers, so you can sit at the server select screen while doing it if you want.
I've never changed CD keys while in a server, and I highly suggest you don't in case it kicks you or something.
idk, I just don't see why you should go through the trouble of downloading anything or switching a 5x5 key to a 4x4 key.