So far, upgrading from one development 1.9 release or 2.0.0 release candidate to another was not officially supported. Only unofficial patches and only for MySQL were available.
Starting with 2.0.0rc3, all database upgrades for all supported databases will be fully supported.
This should be great news for those who would like to do extended testing, as it will allow to create more complex test data and keep it while upgrading. Additionally, this would be useful for people who would like to start with a test deployment, and are cautious about redoing all of their configuration for further 2.0.0 release candidates or the final 2.0.0 release.
This commitment is largely because of the Zabbix maintainer for Gentoo, MattM, who was asking for that on the Zabbix IRC channel