Welcome to the Community,
Afaik you can upgrade/downgrade licenses in your My VMware portal as needed. The Essential Kit comes with 1 license for vCenter Server for Essentials and 6 CPU licenses which can be used for up to 3 hosts with a maximum of 2 sockets each. So what you can do is to downgrade only e.g. 2 of the CPU licenses to get a 4.x license key for the current host and leave the other licenses at v5.
Regarding Veeam, any paid license should do, as these licenses unlock the required API. However, I recommend you double check with the Veeam documentation/system requirements.
André