Hello.
About the licenses :
you can still use your licenses, even if your subscription expired or you can't renew it.
The subscription entitles you to support and to future updates and patches that will be released in the time period that you have your software subscription in effect.
You can downgrade or upgrade licenses in my vmware.
In most cases licenses can be activated or changed online without problems.
The Essentials Plus Kit Each kit consists of six processor licenses for vSphere and a license for one instance of vCenter Server for Essentials
The vCenter Server for Essentials is limited to 3 hosts with vSphere Essentials and vSphere Essentials Plus.
Attached link with more details
https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/vsphere/vmware-vsphere-pricing-whitepaper.pdf
On the new server you can install the same version of VMware vSphere you are using (if supported on the server) or a new version. Finally if you connect the new server directly to the Storage or through the SAN then you can migrate the VMs online.
By installing VMware vSphere you have an active 60-day full demo license.