One of the important settings to consider when creating a new Virtual Machine in vSphere is the VM firmware, which can either be BIOS or EFI and can be configured under VM Options->Boot Options->Firmware. After selecting the desired guest operating system (GOS) in vSphere, the system will default to a recommended firmware type and can also be overridden by the user. Ultimately, the selection of the VM firmware should be determined by what your GOS supports.
If you ever need to change the VM firmware, you typically will need to re-install the GOS because it does not understand the new firmware change (just like in a physical server) and more than likely the GOS will also not boot due to this change and this is the existing behavior from GOS point of view.
For a net new VM creation, prior to vSphere 8, if you had configured a VM using EFI firmware and you have not installed a GOS and realized that you had made a mistake and needed to change the VM firmware to BIOS, you could easily do so using the vSphere UI or API and then install your OS. In vSphere 8 and specifically when using the latest Virtual Machine Compatibility (vHW20), you can not just switch the VM firmware after the initial VM creation, especially if you had started with EFI firmware and wish to change it to BIOS.
In doing so, you will come across the following error message:
ACPI motherboard layout requires EFI. Failed to start the virtual machine. Module DevicePowerOnEarly power on failed.