Use svn move to move the contents of the old trunk somewhere else and rename the branch to trunk afterward.
Note that copy and move in svn work like file operations. You can use them to move/copy stuff around in your repository and these changes are versioned as well. Think of “move” as “copy+delete”.
[EDIT] Nilbus just notified me that you will get merge conflicts when you use svn move.
I still think that this is the correct approach. It will cause conflicts but if you merge carefully, chances are that you won’t lose any data. If that bothers you, use a better VCS like Mercurial or Git.