Add or Upgrade ESXi 5.5

Hi,

I have a small VMware vSphere infrastructure 4.0

It is composed as follows:

1 datacenter with 1 cluster, 2 esx 4.0 and about 10 vms.

I bought 3 server esx 5.5u2 and now I would like to raise my virtual infrastructure.

The best solution is to upgrade the current environment or perform a new installation?

If I upgrade the current environment i can add the new server esx 5.5 in to the esisting environment?

What is the best practice?

Thanks.