I had a Debian 11 system on an HP desktop. I followed How to Upgrade to Debian 12 from Debian 11 to upgrade it. This worked well, though there were a few glitches.

There was an option to preserve the existing grub configuration. Having Ubuntu 22.04 and Windows 10 on the system I wanted to keep it. I thought I had clicked it, but when I rebooted Debian was the only OS that showed. So I looked at How to Update Grub on Ubuntu and Other Linux Distributions and ran sudo update-grub. This ran without errors, but gave the warning that

os-prober will not be executed to detect other bootable partitions.

so it was not doing what I wanted. An article on the ArcoLinuXL -XS -D -B Forum explained that I needed to add the line

GRUB_DISABLE_OS_PROBER=false

at the end of the file /etc/default/grub before running sudo update-grub. This did the job. I supposed I could have used my boot repair disk, but that would have involved more work.

I soon noticed that I could not get any sound from FireFox (ESR) on my upgraded system. Looking at firefox and no audio suggested that I run sudo apt install apulse, which I did. I closed FireFox completely and reopened it, after which FireFox sound worked fine. This was a really obscure error and fix. I wonder it will happen on a fresh Debian 12 install. I will find out soon enough.