Jens Ropers wrote:
We don't actually have to move anything
If you want to introduce a new namespace (or namespace alias) called "X", for example, you *do* have to move all the pages whose title begins with "X:", or else they will become inaccessible.
This is because an article "WP:SB" is actually an article with the title "WP:SB" in the article namespace, while an article "Wikipedia:SB" is actually an article with the title "SB" in the Wikipedia namespace. If you introduce a new namespace (or namespace alias) called "WP", the article [[WP:SB]] will still be in the article namespace, but the server will start looking for it in the new namespace and not find it.
Timwi