Datatypes and field names are stored in the UNK table. There is just one entry per field name, so it's critical not to use the same field name more than once in an application with different datatypes.
You need to rebuild the UNK table, as I blogged here http://www.intec.co.uk/full-text-search-musings/
Note, it must be an offline compact, as Duffbert says here http://www.duffbert.com/duffbert/blog.nsf/d6plinks/TDUF-5SMHV4. If anyone is in the database when you do the compact, it will fail and the UNK table will not be rebuilt.