If you are planning to update to a newer version within the current major version of TARGIT, then you do not need a new license key. E.g. if you are updating from a 2018 Update 1 (2018.1) to a 2018 Update 3 (2018.3) you do not need a new license key.
Approximately, once a year TARGIT will release a new major version, and if you are planning to upgrade from one major version to another major version, then you will need a new, updated license key. E.g. if you are updating from a 2018 Update 4 (2018.4) to a 2019 RTM (2019.0) you do need a new license key.
To get a new, updated license key, please get in contact with your TARGIT account manager. Your account manager information is available when you log on to the TARGIT portal at http://portal.targit.com.