You said that this is spare server, so the question is whether you use any of the advanced features which require a paid license!?
If none of the features are used (and I think this is most likely the case), register for the free Hypervisor edition at Download VMware vSphere Hypervisor for Free, and enter the license key you get there in the current ESXi host (for the steps to do this read the "Licensing a standalone ESXi host" section at VMware KB: Licensing ESXi 5.x and vCenter Server 5.x). No need to reinstall ESXi nor downtime required.
André