On the migration to use bigint: added that this can take hours and will increase the database size substantially. (Example with a real database running on a linode: 40GB => 200GB (without analytics) and over 5 hours). People (like me) might think that the startup is stuck, since there is not message in the tomcat log that this is happening.
The Server base URL system setting has changed from being a system setting to a dhis.conf property. This is done because the server base URL typically follows the server and not the database. The setting key is server.base.url. See install guide for details.
Most metadata database tables now use 8-bit int data type for the primary key column. This migration happens automatically. For the migration to succeed it is recommended to increase the PostgreSQL max locks per transaction setting such as max_locks_per_transaction = 96. See install guide for details. Note that this migration can take several hours, and that the database size may increase substantially during the process.
For Tomcat versions after 8.5, it is necessary to include a relaxedQueryChars="" attribute to allow for brackets in URLs in the Connector element in server.xml. See install guide for details.
The (unused) association between Program and DataApprovalWorkflow is removed.
The (unused) representative association between TrackedEntityInstance and TrackedEntityInstance is removed.
A Geometry (Geometry, 4326) field geometry is added replacing the coordinates and featuretype fields for OrganisationUnit.
A Geometry (Geometry, 4326) field geometry is added replacing the coordinates columns for OrganisationUnitGroup.
The trackedentitydatavalue table has been replaced by a PostgreSQL JSONB object/column in the programstageinstance table.
The Event capture app is removed. It is replaced by the Capture app which offers all of its features and more.
Remember to drop SQL view from Apps > Data administration > SQL views before upgrading to avoid database integrity issues during the upgrade process, then recreate them after the upgrade is completed.