Cannot create column with type "TIMESTAMP WITHOUT TIME ZONE" in PostgreSQL

Instead of using the tag and switch completely from XML to sql, you could modify the resulting generated SQL using the tag which is valid across the whole changeset: http://www.liquibase.org/documentation/modify_sql.html

for example in your case you would have this:

<modifySql dbms="postgresql">
    <replace replace="WITH" with="WITHOUT"/>
</modifySql>