Don’t… just don’t. That’s my advice. There are several fixes & new features, but few of them matter to the average individual user, and it’s buggy. Here’s my experiences, fighting with it for [far too] many hours…
If your VMs use MBR disks, they should work OK, though you’ll likely have to fiddle with the display resolutions and scaling, since this doesn’t work as well or the same as before. I had a few problems with the display until I set the resolution in Win7 & Win10 – setting it using the VirtualBox menu, which I’d done previously, doesn’t seem to work, and those prior settings gave me some problems. If however, your VMs use GPT disks &/or UEFI booting, you’re going to have [big] problems.
VirtualBox 7 adds TPM, which seems to work fine, & UEFI / Secure Boot, which doesn’t. Under the settings for a VM, under System, you can toggle TPM on/off, and for some reason have your choice between v. 1.2 & the officially required 2. There are separate check boxes for UEFI & Secure Boot, though there doesn’t *seem* to be a difference whether you turn on Secure Boot or not – the results seem to be the same. In my experiences… a VM on a GPT disk will not boot unless it’s set to UEFI, VMs that used to boot using UEFI will not boot in V/Box 7, and neither Windows BCDBoot nor EasyBCD will create boot files that will work. You can install Win11 without using tricks / hacks and copying the boot files from that VM will not work [further down I’ll talk about what did]. Additionally, myself & others have had problems with Win11 crashing repeatedly when you try to install the version 7 extensions – I had similar problems with the paired, UEFI booting Win10 VM [in both cases uninstalling the offending V/Box driver in Device Mgr. allowed the installation to work]. In Win11 the flyout display, e.g., right clicking the Start Button is much too transparent – Windows settings did not help – while the paired Win10 suffers from a few minor display glitches, e.g., buttons where the text is not visible in dark mode. And in the paired Win10 & Win11, the paravirtualization network driver I had been using stopped working – I had to download & use a newer version. github[.]com/virtio-win/virtio-win-pkg-scripts
Previously UEFI booting was officially an experimental feature, though it worked. When I originally set up my Win11 VM I started by converting a Win10 Insider VM to UEFI boot and converted the MBR disk to GPT. Then I performed the Win10 to Win11 upgrade, changed the IDs for the VHD [Virtual Hard Disk], and using a copy of the original Win10 VHD, used EasyBCD to set the VM to dual boot – when I fire up the VM I get a boot menu letting me select either Win10 or Win11. I assume Win11 can be made to work on a MBR disk, but I could be wrong – I’ve never tried it. I simply figured that sooner or later the kind folks developing VirtualBox would add the UEFI / secure boot and TPM that Win11 *officially* requires, so it would be easier to setup Win11 that way from the start. And it’s been working fine that way since Win11 debuted.
If you are going to turn on V/Box 7’s UEFI, the Main thing you need to know is the location of C:\ Users\ [UserName]\ .VirtualBox\ Machines\ [VMName]\ [VMName].nvram – I fear you’re going to need to delete that file, a LOT. If/when the Windows VM won’t boot, the first thing you do is delete that file, then check the VM’s settings. If/when Secure Boot won’t turn on, with a prominent error message, delete that file. If you boot to an ISO, then change the ISO or turn off booting to that ISO in settings, and it still boots to the ISO, delete that file. Let me put it this way: I kept a File Explorer window open to that folder I deleted that file so often. Finalizing things after getting everything working last night, I renamed the VHD Win11 uses, & had to delete that file twice getting the VM to work with the renamed disk file.
Secure Boot works by matching the files the BIOS starts [in order to start Windows] with a list of known good files, to try and prevent it from running malware. With real hardware like a PC or laptop that list is stored in the BIOS memory – in V/Box 7 it’s apparently stored in that .nvram file, along with apparently [lots of] other stuff. Under the VM’s Settings -> System there’s a button to Reset keys to default – click it and you’ll see a warning message that it could make the VM unbootable… BELIEVE it.
Now today, the logical thing to do if you want to run Win11 in a V/Box VM is to turn TPM & Secure Boot on in a new or existing VM, set the OS to Windows 11, attach / mount a Win11 ISO, then install Win11 fresh. If you use an existing VM that uses a paravirtualization network driver, set it to an Intel network adapter – you can install the latest driver version afterward, setting the network adapter to paravirtualization, attaching the virtio-win-0.1.221 [or newer] ISO, starting the VM, then updating the network driver in Device Mgr., having it search the mounted DVD. The Win11 install really doesn’t take long. Myself, I hate setting up a new Windows install, and I’m stubborn, so I went the harder route, figuring out how to get my existing VMs working.
Like when I got my 1st NVMe drive, using BCDBoot or EasyBCD to create the boot files on the FAT32 partition did not work. The Macrium Reflect rescue ISO would not even boot – the Paragon HDM 17 rescue ISO would, but it could not fix the boot files [BCD]. I got the same results copying the boot files [partition copy]. Using the boot repair tool on the Win11 setup ISO didn’t work. The only thing that would boot was a freshly installed copy of Win11. I deleted the FAT32 boot partition on my existing Win11 VHD – backed up the Windows partition, deleted all the partitions, restored the backup, then resized the partition to take up all the available space – and adding that VHD to a new Win11 install using Easy BCD to create a new boot menu, I could get that old copy of Win11 running. Next, I restored the backup from that old copy of Win11 over the new copy, but it would not boot until I used the Paragon ISO to correct the boot files. Then I attached the VHD for the paired copy of Win10, using EasyBCD to add it to the boot menu – that worked, but again would only boot after booting to the Paragon ISO & correcting the boot files.