I have a client that left the 3CX family for a nother product. I would like to use the licence key for another customer. if I transfer the key, will I be able to generate a new FGDN?
If you are talking about the FQDN, make sure it has not been released already by looking at the FQDN field. If it does not show an FQDN and it instead states "Released" then the License Key is currently not bound to an FQDN, meaning that using it during a new 3CX Installation will allow you to choose a new FQDN.
If you are referring to the License Key's ownership, you will need to click on "Transfer Ownership" on the left hand side of the registered email of this Key. If you do not see this option, it most likely means that you have not accessed the 3CX Portal using the account who is the owner of the key.