When you upgrade your FaxMaker license it is updated to the most recent version. This works for many customers, but not for some as they have restrictions on the version they can use due to various reasons and having the license in the previous version is needed. Example, need to use version 19 rather than the current. This article gives instructions on getting the new license key downgraded to the version required.
Verify your current version using the finding your FaxMaker version article.
Once you know your version you can then open a ticket with GFI Support and provide the license key and request the key be downgraded to the version you specify.
Once the downgraded license key is received from GFI Support, it will work in your FaxMaker system of the same version.