If you cannot expand the cluster then there is only one method left and create a new cluster and migrate the vm’s to the new cluster. 
 

Or install the new cluster with AHV and use Nutanix Move to migrate to the new cluster. Delete the old one and safe some money on the vmware licenses. 

I never made a migration like this.

If i create a new cluster with Esxi (i tried it but i am not totally convence by AHV at this time), to migrate VM’s i need to mount in the new Esxi cluster the actual Nutanix Datastore ? Forget VM’s in actual vCenter side and register them on the other vCenter from de mount datastore, is right ?

Nicolas 

There are 2 concepts to consider, the Nutanix Cluster and the vCenter Cluster.
The Nutanix Cluster can handle 6x nodes without a problem (even with Starter license), the problem is on the Vcenter License that is limited to 3x nodes.

If you have the capacity to sustain 1 nodes off-line, then you can simply add your new Nutanix nodes to the cluster (bringing your Nutanix cluster to 6x nodes), you just do a Foundation and deploy AOS with ESXi on it (you don’t need to add it to the vCenter, only install ESXi).
When your 6x node Nutanix cluster is ready (all nodes sharing the same storage = same cluster) you simply put one of your G5 models in maintenance mode on the vCenter, Power-off and you remove it from vCenter and you can add a new node to the vCenter to replace the old one. Rinse and Repeat until you have 3x new nodes on the vCenter, then you can go to the AOS and remove the old G5 from the cluster.

Also keep in mind that your vSphere version is no longer supported so you might eventually need to upgrade your licensing which can help with the operation as well.

Hello NBorba,

Do you mean I can expand the Nutanix cluster with my 3 new nodes without them being registered in actual vCenter ?

I seem to have seen that the new nodes must be registered in the vCenter BEFORE expanding the Nutanix cluster otherwise the procedure fails.

Are you 100% sure about this method ?

Nicolas

Is it possible to request a temporary (trial) license from VMWare for the vCenter and ESX during the migration ?

Correct. I’m 95% sure the Nutanix Cluster expansion only needs to connect to the ESXi.  
Simply run the Foundation and deploy ESXi, ensure vmkernels (management and vmotion) are configured etc… since you are running standard virtual switchs you can do everything on ESXi.
Once your esxi is configured you can add the node to the Nutanix Cluster, it will basically extend the cluster storage so be carefull when rebooting the new nodes because they will have a CVM running and they will be contributing to the overall Nutanix Cluster Storage.
In regards to the Licensing, I doubt that you will be able to pull a trial for vSphere 7 since the EOL is in 3 months from now and Broadcom doesn’t want to sell this licensing model anymore. 
I would recommend you to look at renewing your license or as mentioned above think about moving to AHV since you are already paying for it :) 

I made a trial license request to Broadcom a few minutes ago.

It will come probably in vSphere v8.

I will see if i can convert it to vSphere 7 else i will try your method.

I had already prepared my 3 new nodes with Esxi 7.

Thanks for yours advises 

Nicolas

New twist after obtaining "trial" license keys to bypass the 3-host limitation of the vSphere Essential license, I am now faced with another constraint not anticipated by our Nutanix supplier : my new nodes are full SSD while the old ones are not...

As a result, it is impossible to expand the current cluster with these 3 new nodes.

So I have to create a new cluster.

I think I will be able to migrate the VMs from one cluster to another, but do you know how to ensure that Veeam Backup does not consider the VMs migrated to the new cluster as new VMs to maintain the continuity of incrémental backup and replications ?

Nicolas