Open the vmx-file in a texteditor.
Search for the line
firmware = "efi"
and delete it.
On next boot the VM will start with a legacy BIOS but will very very likely fail to boot.
Changing the firmware on an existing VM is not trivial - if you really need to change that the best way is to create a new VM from scratch.
- Select the VM
- Right click settings
- select Options tab
- Select Advanced Button
- on the right side of pan select the firmware type you need by selecting a radio button (BIOS/UEFI)
One reason would be building a windows image that boots UEFI, but the software used to capture the image only boots over the legacy network. Its fairly simple to change this as needed.