It would be a lot easier to update IMPEX than to downgrade your ubbthreads installation.
Although, there haven't been much radical changes in the database between 6.4.4 and 6.5.
The Categories table changed and all sticky posts have been moved to the Accouncements table.
All other changes just added some new tables, table fields or indexes. So using a 6.4. importer should work, if you just downgrade the categories table and copy the annoucements back into the posts table.
