Code:
Guide by CrAzY of HSForum.net
Here is a very simple way of downgrading to 1.0, no repair needed.
Steps
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Step 1) Go to your World of Warcraft Directory (Normally C:\Program Files\World of Warcraft)
Step 2) Go to The Data Folder (C:\Program Files\World of Warcraft\Data)
Step 3) Delete These Files: patch.mpq, patch-2.mpq, patch-3.mpq (If patch 2 or 3 aren't there, that's fine)
Step 4) Place this in your WoW directory (Step 1): http://www.megaupload.com/?d=Q0014FUL
Step 5) Replace that wow.exe with your current one
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Your done, now you may patch.
I uploaded that wow.exe, Its just from the base.mpq (In your data folder) which I extracted with winmpq.
Please when copying this guide, please leave who wrote it and from what site.
As far as I know you just have to go back to vanilla, as I think it writes over/deletes the previous patch. However, I could be very wrong.