That may take a while to do depending on the number of
extents that have been created.


one of the best ways I use is to start up the db with a very small buffer cache (say 100 db block buffers) and make sure you only have 1 DBWR configured. Obviously, the db won't be usable in that type of configuration, but it should help
smon clean things up faster. So, if you have a window of opportunity to try this out, this might be the best thing to do.

There are also 'unsupported' ways of doing this probably such as updating the appropriate data dictionary tables, etc, but this would have to be done with extreme caution of course.