The clean way to do this is using svnadmin
to dump out the whole repository using
svnadmin dump
Then create a new repository with the trunk directory at the root, and reload the dump with
svnadmin load --parent-dir trunk
If you do an svn move then that will mess things up if you ever update back to a revision before the move, since the files will move back to their previous location, which is probably not what you want.
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