A MATTER OF PRINCIPLE
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Will the hardware come with you?
Discontinued CPUs block the install, deprecated ones start a countdown, and the network cards fail quietly. Then the choice between import, converge and side by side, which is not the binary most people treat it as.
You are estimating from the wrong number
Host count used to predict upgrade effort and no longer does. A VCF 9.x upgrade scales with component count, which barely changes with estate size, so the smallest estates carry the worst ratio.
Your estate no longer has a version number
VCF component patches are no longer synchronised and no single page enumerates a patch set. What the 12 October release actually contained, the order it has to be applied in, and what that does to a change record.
Fleet operations without a fleet
VCF Operations certificate management now reaches standalone vCenter 8.0 Update 3 and above, and VCF 5.2, without converting those estates to VCF. What 9.1.1 added, the CA options, and the licensing line that decides whether you can use it.