The 40k servers are probably made up of multiple redundant vSphere clusters with failover. You simply take one of those redundant clusters and migrate one half of it over. Then the other half. Then duplicate that process. As you build more compute in the new stack, you can decomission more and more of the old stack and convert it. The transition would progress like a cascade, with larger and larger groups of clusters being migrated at once until you're left with the one-off, ad-hoc, weirdo clusters at the end that need to be manually migrated (usually with great effort).

The actual hardware servers are clustered together into pools of resources. The pools are where the VMs live. The bigger the new pool becomes, the faster you can empty the old one. So the migration starts very slowly, ramps up quickly, and then tapers off.

> You simply take one of those redundant clusters and migrate one half of it over.

For that half you are migrating, you are essentially operating without redundancy. If these are serious production workloads, the tradeoff is not as simple as you make it seem.