so when you say 'down' you really mean ( to me ) UP ?
ie:
currently
http://yoursite.tld/ubb <-- ubbthreads install
desired
http://yoursite.tld <-- top level
if so, then no fresh install, but just change config.
if other way where you want to move it from top down to /ubb dir, then no fresh install, but just change config too.
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also for WP, i always install that in a secret dir like
http://yoursite.tld/wpzeeee <-- ( make up the wpzeee )
then with the permalink structure the way WP can be, you map top level stuff to the hidden directory and mask off the 'wpzeeee' part of the url. the users simply don't see that
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this way you really can have 2 top level looking installs even though the wp is located in a hidden dir
