svn switch --relocate http://textpattern.com/svn/repos/current/textpattern http://svn.textpattern.com/current/textpattern
Where the old url is http://textpattern.com/svn/repos/current/textpattern
And the new one is http://svn.textpattern.com/current/textpattern
To see if it worked, do this:
svn info | grep URL
And you should see something like this:
URL: http://svn.textpattern.com/current/textpattern
(source: Switching a Working Copy)