PG 8.4 and pg_autovacuum functionality

Hello,

Since pg_autovacuum no longer exits on PG 8.4 and it seems that one
now needs to provide the storage parameters during CREATE TABLE or
later on with an ALTER TABLE.
Will that ALTER TABLE block anything going on that table until it's
finished ? I assume not since no table data is actually being rewritten.

Thank you,
Marcelo