Full Release Notes with installation information are located here: 17.10 Full Release Notes
When you upgrade from 17.04 to 17.08 the Audit index is not lost. The logged system activity is maintained and new entries are added without problems.
However, when upgrading from 17.08 to 17.10, the system activity is cleared and the index is recreated from 0. Please keep in mind that the ES settings were changed in 17.10.
Who Should Install?
This release is recommended for all 4.6 and later installations. If you have a patch installed to your system a new patch may be required. Please contact email@example.com if think you may have a patch applied as that may be replaced during the update.