1. Download http://ftp.blizzard.com/pub/WoW/other/Repair.zip & the patch corresponding to 1.7 in your language.
All 1.7 full patches can be found at Filefront. You most likely need the English US manual patch
2. Go to *\World of Warcraft\data and delete "Patch.mpq"
3. Extract repair.zip to *\World of Warcraft\ and run repair.exe
4. Repair.exe should say that your install is broken and ask if you'd like to fix it, click yes. Wait for repair.exe to scan your files.
5. Move the patch file to the WoW install you want to downgrade
6. Run the patch file and wait for it to finish. Edit your realmlist.wtf to point to dwow.ath.cx
7. You've successfully downgraded your WoW version!
I've used this method with complete success on my personal computer. If for some reason it fails to work, just uninstall and reinstall. Hopefully that doesn't happen, though. Very Happy
Eh....