There may be special circumstances where you would choose a migration or fresh installation to achieve the upgrade results you want.
For example, if you are on an older Teradata Database version that is not the minimum level for a specific upgrade, a standard database upgrade will not work. Your system will require special handling, which could include a two-phase upgrade or a data migration.
If you want to move to a different operating system as part of the upgrade, your system will require a data archive, a fresh installation of the operating system and the Teradata database, and a restore of the data.
Consult with your Teradata Services Consultant to determine the upgrade process appropriate for your system and target upgrade version.