Apache Roller trunk (i.e., 5.1-SNAPSHOT) will automatically upgrade any Roller database that was part of an official release from Roller 3.1 onwards during its install process. Follow the Apache Roller installation guide for more information on this process, including making sure that you do a safety back up of your database prior to running the install.
However, for those who have earlier deployed unreleased 5.1-SNAPSHOT versions, neither later 5.1-SNAPSHOTs nor the future release 5.1 will be able to automatically upgrade your SNAPSHOT database, which will create a incompatibility issue if database changes occurred in development since the snapshot version you deployed. This will necessitate some manual database upgrade changes prior to moving to the later version of Roller.
To determine if any manual database changes are needed, check the revision number located on the footer of the Roller login page, i.e., the (rXXXXXX) value at the end of the "Powered by Apache Roller Weblogger Version 5.1.0-SNAPSHOT" string. Then, using the source control repository of the createdb.vm file, do a comparison (using the "select for diffs" links) between versions of the file immediately preceding that version number and either the latest version of the file (for a future 5.1-SNAPSHOT) or the version just before but no later than the release date of Roller 5.1.
For example, here is a comparison between March 2014 and May 2014 database instances. Note the createdb.vm does not provide the actual SQL statements needed for your particular database instance (as that will vary depending on the database you're using) but just the pseudocode SQL informing you which tables and columns need deletion, creation, modification, etc. Check your database's manuals for information on the necessary SQL statement syntax for table alterations, and of course back up your database first.