How do I tell an svn client that's already checked out a working copy to use the new URL given that nothing apart from the URL is changing?
svn switch --relocate <old URL> <new URL> <wc path>
is the command you want - details at http://svnbook.red-bean.com/en/1.5/svn.ref.svn.c.switch.html
(note that "svn switch" on it's own is insufficient).
HTH,
Greg