Yes. When you install an updated version of Genstat the installation process will create a new folder eg. Gen21Ed, Gen22Ed, Gen23Ed, etc. This keeps all of your versions separate (newer versions do not overwrite older versions).
As long as your licence is still valid for a previous version it will continue to work. For example, if you have Genstat 21 running under the older Flex licencing system, you can also have Genstat 22 and Genstat 23 (using the newer RLM licencing system) running simultaneously on the same device. When you run an older version you will be prompted to upgrade but can continue to ignore this prompt and run the software until its licence expires.