Houdini 21.0 licenses will be able to open Houdini 21.0 and older (20.5, 20.0, 19.5, 19.0 etc).

If you are running a license server locally on your computer and wish to install older Houdini builds alongside Houdini 21.0, ensure to not install the license server for the older versions. The older Houdini versions will use the 21.0 licenses and license tools.