Especially if you're using Subversion 1.4, see this answer for an alternative:
svn: replace trunk with branch.
Otherwise, move is the simplest way to achieve the restructuring.  As @JoelFan said, move will work on directories, so you can perform this operation without a working copy:
svn mkdir --quiet --message "Restructuring" http://svnhost/svnrepos/trunk
svn mkdir --quiet --message "Restructuring" http://svnhost/svnrepos/tags
svn mkdir --quiet --message "Restructuring" http://svnhost/svnrepos/branches
then, appropriate move commands for your sources:
svn move --message "Restructuring" http://svnhost/svnrepos/dir01 http://svnhost/svnrepos/trunk/dir01
Note: If the existing repository has a structure like:
/repos
    /projectA
        /branches
        /tags
        /trunk
    /projectB
        /branches
        /tags
        /trunk
I'd suggest leaving it that way - it would make separating the projects into separate repositories very straightforward if for some reason you need to do so.