I was able to switch by following these steps. I was looking to upgrade my Win10 VM to Win11 and needed to change to OVMF and add TPM.

1. Make a backup or snapshot of your vdisk file ... just in case ;)

2. In windows 10 (1703+) there a new tool added, mbr2gpt

    Open powershell as admin

    Run this command

        `mbr2gpt /validate /allowFullOS` <-- if ok then

    If the above command validates successfully, run this next command

        `mbr2gpt /convert /disk:0 /allowFullOS`

    If the conversion is successful your win10 VM disk is prepared for EFI boot

        shut the VM down.

3. Change BIOS from SeaBIOS to OVMF UEFI

4. Create the EFI disk 1. Make sure Pre-Enrolled-Keys is checked 
5. Create a TPM 2.0 disk 
6. Boot up your VM

After these steps you are able to download the Windows 11 Installation Assistant from https://www.microsoft.com/de-de/software-download/windows11