The problem here (which is also the problem with any script on /opt/opennms/bin that requires Java) is that in order to use hibernate, the all Spring infrastructure must be initialized, including the DB Connection Pool. That means in, order to execute something with OpenNMS up and running, PostgreSQL must support the double of connections configured for C3P0 (which is the case of the upgrader if you do that, the current upgrader doesn't have that problem because it uses plain JDBC).

It would be nice if you can replace the DB Pool size dynamically in order to avoid this problem.