GeneXus licenses, though granted for a specific GeneXus version, can be used in prior GeneXus versions.
When authorizing a GeneXus version, you are able to use previous versions without the need to authorize or transfer the licenses between them.
So, when you get a GeneXus 18 license, you can use that with GeneXus 17, or even prior versions, such as GeneXus 6.1.
However, in certain versions, you need to install some compatible upgrades, as follows:
Note: If you use centralized licenses (installed in a server), you will have to install a GeneXus Protection Server that can hold license keys for the latest version of GeneXus. Supported GeneXus versions are listed here: GeneXus Protection Server