Database schema changes are manually applied during DSpace upgrades. In a multi-release upgrade, several SQL scripts must be applied, in order. This is fragile and error-prone. We should pull this into the 'ant update' target so that it can be done consistently and at the right place in the process.
It may be useful to consider Rails "database migrations" as a model here, and indeed there is at least one Java-oriented DB migration package: http://flywaydb.org/