Upgrade Roadmap 8.1 to 9.1

The first thing is know the correct upgrade path:
Vertica 8.1 to 9.0
Vertica 9.0 to 9.1

++Platform Requirements and Recommendations:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/InstallationGuide/BeforeYouInstall/PlatformRequirementsAndRecommendations.htm

Once you complete the before task you can proceed with upgrade:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/InstallationGuide/Upgrade/RunningUpgradeScript.htm

++ Before upgrading to 9.1 or higher, you are strongly urged to check your database for unsupported projections. If the upgrade encounters these projections, it is liable to fail. 
You must then revert to your previous installation:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/InstallationGuide/Upgrade/PreUpgrade/IdentifyUnsupportedProjections.htm

Here the pre-upgrade-script:
https://vertica.com/pre-upgrade-script/


++Checking Catalog Storage Space, vertica recommends have at less 40% of free space per each node:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/InstallationGuide/Upgrade/PreUpgrade/CheckCatalogStorageSpace.htm


++ Verify License Compliance for ORC and Parquet Data:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/InstallationGuide/Upgrade/PreUpgrade/VerifyORCandParquetLicenseCompliance.htm

++Perform a full backup of the database, this is a must, you should have a backup in case of something wrong during the upgrade:
https://www.vertica.com/docs/9.1.x/HTML/index.htm#Authoring/AdministratorsGuide/BackupRestore/CreatingFullAndIncrementalBackups.htm

Now, 9.1 have new features and some reported bugs were fixed, to check them you can go to the below link:
https://www.vertica.com/docs/ReleaseNotes/9.1.x/Vertica_9.1.x_Release_Notes.htm