I'm in the process of investigating upgrading from ESXI 4.x to 5.x. I would like to know if new hosts were built on the esxi 5.x platform could we later vmotion our current 4.x VM's to 5.x and then upgrade the current set of 6 hosts.
Also is there a cli script available to upgrade all the components.
Yes, you can do this, but you would need a 5.x vCenter to manage the whole thing. It would be just as easy to upgrade your existing vCenter, vMotion everything off of one host, upgrade it, vMotion everything back, lather rinse repeat 5 more times.
Yes, you can vMotion from a 4.x host to a 5.x host - upgrading a cluster should be done by putting hosts in maintenance mode, upgrading them, then moving VMs back to them. Fresh hosts will also work.
There is not a script that can handle the process, because it's not that simple. You need to go through the documentation and work out a plan - upgrading the vCenter first, then the hosts. See here.