vSphere 4 to 5 License Upgrade

Hi,

We are planning to upgrade as below;

1 vCenter 4 server to vCenter 5

3 ESX 4.1 hosts to 5.0

Currently they are licensed for vCenter Standard 4 and vSphere 4 Enterprise Plus

We also have vCenter Standard 5 and vSphere 5 license.

I want to know WHEN and HOW to upgrade vCenter server and Hosts LICENSES. Not the vCenter server and Host upgrade path from 4.1 to 5.0

thanks.