I work in automating conversions off VMware and 40,000 VMs is just a lot of data to move. We could probably do 500-1000 / day which would be 3 months, but that would be best case, and there's a lot of prequalification where you examine classes of VMs to check what software they're running and identify the unsupported / difficult cases. That planning would add extra months.

In some cases you can do zero copy conversions, so downtime can be done in a few minutes, but it relies on the customer have very particular storage configurations (NetApp basically). In other cases there can be significant downtime that needs to be scheduled. I worked one case where the customer shut down several production lines over a number of weekends so we could convert the workloads. (Everything was meticulously planned, along with fallbacks that thankfully we did not need to use.)

Some things you don't convert at all. Databases generally get replicated at DB level to new hardware. Single-purpose appliances need to be reprovisioned by going back to the vendor and asking for a KVM equivalent.

Then there's all kinds of craziness, like we had customers who rolled their own backup solutions where we had to add special cases to the software to detect and ignore the backup partitions. Or people running Windows 95 or RHEL 3 (for real!) where there are no virtio drivers and we don't certify the hypervisor so it requires support exceptions. At this point people have been using VMware for nearly 30 years, there's all kinds of crazy legacy.