5.0 and 5.5 should use the same keys. So start by adding the new license key to your vCenter server so you are able to manage more than 3 hosts.
Next step would be to update the vCenter server to 5.5 so it can actually manage 5.5 hosts.
Afterwards you would update the hosts.
Host licenses can also be replaced at any time during this process, as they are also version 5.x specific.
Theoretical you could even stay on 5.0 and just replace the licenses if they are not some kind of special license keys.