Below are some important features and considerations worth highlighting when upgrading to 9.3 and higher.
If you are coming from 9.1 or earlier versions, all timestamps in the database will be converted to UTC time. This creates thousands of changes which can cause the next sync to be much longer.
All Ektron Windows Service endpoints are TLS secured in 9.3. This means that the service is incompatible with sites earlier than 9.3. In the past you could upgrade just the Ektron Windows Service and not the sites. For 9.3 we recommend you upgrade both at the same time. eSync certificates are also expiring on 1/1/2019. See the following for more on the secure endpoints and expiration date.
Esync and version 9.3 - Considerations and issues for new and older versions
Solr - PostgreSQL
You must upgrade Solr when upgrading the CMS to 9.3, so you may want to consider changing Solr to use PostgreSQL - a new database option in 9.3. CMS sites with large numbers of content items can benefit from increased crawl performance by changing from the file-based HSQLDB to PostgreSQL.
Be sure to checkout the official documentation and release notes as well.