Once the hypervisor has been upgraded, you can start migrating virtual machines on the machine; but not everything has been upgraded to Version 5.5 yet. VMware Tools running inside the virtual machines can be upgraded as well to get the most out of the new hypervisor. The virtual machines, however, can continue to run with the older version of VMware Tools.
VMware Tools include the following:
VMware Tools for all supported guest operating system types are packaged with ESXi. For Linux, they are also available as VMware Operating Specific Packages (OSPs) for download from the repository URL: http://packages.vmware.com/tools ...