If you already have WC v1.23 , check out this new tool that lets you switch from versions 1.18, 1.19 ... all the way to 1.22 in just a simple installation of less than 7MB: http://www.dotapod.com/thread-5443-1-1.html
Older information:
Okay.. this might be useful for a multitude of reasons. Maybe you need to watch your replays on 1.21 or 1.20 or maybe you have friends who are still using 1.21 (for some reason or other) and you want to play with them. You could also use this to backup your current version in case you run into some problems while upgrading and want to rollback to the previous version.
Step 2. If you haven't updated yet and want to create a 'restore point' of sorts, Open the program and click the Store current version button. A box will pop up and ask you what version this is, they won't have 1.21 or 1.22 so just select any version number you want and click make zip package, you can rename it later. For this example, i'll select 1.00.
Step 3. Inside your Warcraft Version Switcher directory, open the folder called wvs, and you'll find the zip file the program generated with your core war3 files. If you selected 1.00, the file should be called something like TFT_Version 1.00.zip. For record purposes, you might want to rename the file to reflect your current version (eg TFT_Version 1.21.zip).
Step 4. Patch as neccessary to 1.22
Step 5. Create another restore point at 1.22 (for your own convenience if you want to change between versions regularly)
Step 6. Use the button Switch version to easily hot swap between versions. You may have to close Warcraft 3 before you switch versions.
Please note that if you change versions, you have to update the record of your warcraft 3 version in zion.
(use the command /z.options, and change the version number as neccessary)