The 16.20 release can co-exist with the 15.10.10.00 release and later versions. The active release will be the last release installed. For example, if 15.10.10.00 is installed, then 15.10.10.00 is the active release. Now, if 16.20 is installed after installing 15.10.10.00, then 16.20 will become the active release. The releases can be installed in any order. Installing 16.20 then 15.10.10.00 makes 15.10.10.00 the active release.
There is an "Uninstall Previous versions" check box on the "Select Features" UI that should be unchecked when installing 16.20 if the 15.10.* (* is 10 or greater) release is not to be removed. That option is also available in silent install. See Silently Installing Teradata Tools and Utilities Products for details.
There is an application named TTUListProducts that can be used to set the active version if more than one release is installed. This application is available on the "Teradata Client 16.20" start menu item. TTUListProducts indicates the current active version and lists all the products that are installed on the system. The difference between the active release and non-active release is that the bin folder will be on PATH. A logoff or reboot will be required when the PATH is updated in order to take effect. This is a Windows restriction. The non-active release products can be used from the start menu item for that release.
A bat file named ttu_1620_PATH.bat is provided in the bin folder that can be used to access the 16.20 release in a command prompt window if 16.20 is not the active release. This bat file will update the PATH to include the 16.20 bin folder.