1. You have 2 Options. Go into your WoW folder,
World of Warcraft>Data>patch.mpq and patch_2.mpq, COPY and PASTE these 2 files Into another folder (This insures that you wont have to re-install Warcraft)
OR
Copy and Paste a second WoW Folder, this isn't necessary. However it Completely guarantees nothing goes wrong.
2. Go back into World of Warcraft>Data>patch.mpq and patch_2.mpq
DELETE BOTH FILES, in many cases, there is no patch_3.mpq If there is, delete it. If not, dont worry.
3.Go into World of Warcraft>Repair.exe run the file. Select all possible checkmarks and hit the Repair button. (SHOULD ERRORS HAPPEN, Paste the copy of patch.mpq and patch_2.mpq. You will not be downgraded, but you wont have to re-install.)
4. Let the Program run its course. Once it is complete, your Folder is set back to 3.0.1
5. Start the WoW Launcher. You should be at version 3.0.1 At this point, allow the launcher to patch WoW for you. Sit back, relax, But once you update to patch 3.0.9 STOP the patch.
Dont bother with downloading all the patches off of website, the Launcher should patch one by one in this order.. 3.0.1 > 3.0.5 > 3.0.8 > 3.0.9 THEN To 3.1.0
6. You should now be downgraded, have a terrific time playing Neverendless! (copied form NE site)