You can't change the value of an AutoNumber field, but then you shouldn't
need to.
The only point of an autonumber field is to provide a (practically
guaranteed) unique value that can be used as a primary key. No meaning
should be assigned to its value: in fact, it's unusual to have the value of
an autonumber field shown to the user. Autonumber fields are not guaranteed
to be sequential: if you start to save a record and change your mind, the
value that would have been used is lost. In fact, if you use Replication to
synchronize multiple copies of the same database, your Autonumber fields
would be random.
