After you upgrade to ESXi 5.0, reapply your host license.

After upgrading, your ESXi 5.0 software returns to evaluation mode until you reapply your license. If you used part of the 60-day evaluation period before upgrading, the time remaining in your evaluation period is decreased by the amount already used. For example, if you used 20 days of the evaluation period before upgrading, your remaining evaluation period after the upgrade is 40 days. See About ESXi Evaluation and Licensed Modes.

You can apply your license using the vSphere Client and vCenter Server. See the vCenter Server and Host Management documentation.