I don’t know if virtualbox uses semantic versioning - if they do - the major release number usually signals breaking changes - no backwards compatibility.
If I recall - the efi image which has changed from v6 to v7.
The change has to do with the requirements of win11 - tpm and secureboot.
Without any way of testing - as there may be implications involvning your activation key - you could try the following
Open the folder with your current win11 vm.
Assuming your current vm’s name is win11
Create a backup copy of the win11.nvram file
cp win11.nvram win11.nvram.bak
Create a new win11 named win11vbox7
don’t install or boot it - the only interesting about the vm is the .nvram file.
Open the folder with the new vm
Copy the new nvram file to your existing vm
Replace your current win11.nvram with the newname.nvram file e.g.
(It froze during the boot process though; but another site that talked about win11 guests no longer booting about the 6->7 VB upgrade recommended turning off 3d acceleration, and that worked for me too. )