Thank you so much for your prompt and valuable answers, they are all good options.
I think that in my case the easiest way is to user ovftool to "just copy" the VMs.
Using ovftool I can test the copy before final switch, and this makes me more confortable.
If any issue will arise, as second option I can use Veeam, my concern is that I only have a 2 host license, but maybe I can connect Veeam to the old host and only one cluster node.
I'm not sure I can use vCenter because I think vCenter 6.5 does not support ESXi 5.1.
Enrico
