When installing ESXi or vSphere on a host which has an existing bootable partition from am older version or perhaps vSan data on existing partitions, some special steps may need to be take .
This is primarily for homelab users, but can serve as a reference for other situations.
During the installation process, the installer will notify you if partitions already exist. In my case I had one drive that was for booting the prior version and 2 vSAN drives which I plan on rebuilding. The installer flags these with the * and # symbols as noted in the key above.
I want to install over my prior boot media, so I select the drive that "* Contains a VMFS partition", but you have to be cautious here. VMFS partition doesn't just mean prior boot media, it could mean a VMFS datastore. Some systems may have been configured with a local datastore, overwriting this partition would cause a permanent loss of data. I am hopeful that you do not need this data and the point is moot, but be vigilant none the less.
Once the drive is selected and confirmed, a new dialog appears if a prior version is detected. You are offered the choice to "Upgrade" or "Install" if upgrading is a supported option.
Hope this helps!