The BIND 9 distribution, and the prepackaged versions too, contains a document
called migration
that contains notes about how to migrate from BIND 8
to BIND 9. The document is very straight forward. If you installed binary packages
it's likely stored in /usr/share/doc/bind*
or
/usr/doc/bind*
somewhere.
If you're running BIND 4, you may find a document called
migration-4to9
in the same place.