After you upgrade to ESXi 5.1, reapply your host license.

If you upgrade from ESX/ESXi 4.x, your ESXi 5.1 software returns to the 60-day evaluation mode period until you reapply your license. See About ESXi Evaluation and Licensed Modes. If you upgrade from ESXi 5.0 x, your existing license applies.

You can apply your license using the vSphere Client or the vSphere Web Client and vCenter Server. See the vCenter Server and Host Management documentation. If you use the scripted method to upgrade to ESXi 5.1, you can provide the license key in the kickstart (ks) file.