This information applies to version(s): 2018, 2, 3, 4
Older versions of Nik Collection are overwritten by default because it is an updated version, and there is no need to have duplicates.
If you choose not to use the new version, and you owned a version of Nik Collection by DxO purchased since the release in 2018, you can always uninstall the new version, then download and install the older version again from your customer account (provided that your previous Nik Collection by DxO version is compatible with your current OS).
If you are a customer from before Nik Collection by DxO was released, please note that these versions are no longer available, so you are responsible for understanding that you are choosing to remove this software from your computer by downloading the trial or installing the paid version. In addition, this warning was available prior to completing the installation of the Nik Collection by DxO.