I have a client's license that is expiring soon. They have downsized their business and I am wondering if it will be possible to downgrade to the new StartUp versions?
Unfortunately, it is not possible to transfer your current license, you can simply sign up for a new trial license from which you will be able to deploy a StartUP PRO version for free: https://www.3cx.com/startup/
Please keep in mind that you will need to reconfigure the PBX as there is no backup and restore capability.