As additional note to scotts answer.

Other questions popped into my mind. My VMware ESXi is licensed for one physical CPU. If I decide to upgrade my server with an additional CPU, do I need to buy a new license or I can upgrade the existing one by paying only the difference?

There is no need for offering an "upgrade" for that task which only make the SKU list much worst as needed.

There is no drawback that you have to buy a new single 1 CPU ESXi license because after you bought it you can combine both 1 CPU licenses to a 1x 2 CPU within the MyVMware portal. After that you can assign this  license key to your 2 pCPU Server.

For sure you can combine only license keys which are the same Edition like your "Enterprise Plus".

Regards,

Joerg