Moving VMs from ESX 4.1 to ESXI 6

Hello,

I'm working on a project to move a 50 VMs from a cluster on ESX 4.1 to ESXi 6.

The VMs will be moved to a new Cluster, so no need to upgrade the old ESX hosts

What I have:
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- A cluster on ESX 4.1 with a Vcenter 4.1

- A New Cluster ESXI 6 with a Vcenter 6

- Netwoking is configured correctely for Vmotion

What I did:
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I installed a new host (spare) with ESXI 5.1, which will play the intermediary host between the ESX 4.1 and ESXI 6. I installed also a Vcenter 5.1.

Question:
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I need to move the VMs without downtime and a way to rollback if things goes wrong.

The VMs are very critical.

I'm looking for a detailled method to succeed in this migration.

Thank you